EDIT: Wow, sorry to just drop this huge block of text into the Waluigi discussion. I was in the midst of typing and brainstorming when all of that was going on, so, sorry if I just kind of 'jumped' in there.
I was typing this up before I had to leave for work today, but long before Smash 4, before Melee came out, I was hoping deep down in my heart that Megaman X would be a playable character. I'd even thought up a full moveset for him at the time, and thought it over again this past week after having played P:M for a while now. I just thought I'd throw out what I'd thought of (though, with Megaman being a playable character in Smash 4, it makes this less satisfying somehow).
The idea with X is that he'd be a character similar to Marth, who had to utilize certain spacing to be at his most effective, and have the versatility of Mario in his attacks in that he could have multiple trajectories on a couple of them, allowing for different mixups in his game. At the same time, he wouldn't be very fast, but not very slow, either, somewhere just below Mario in terms of running speed, but slightly larger than the majority of the 'human' cast members.
Take all of this with a grain of salt, as I'm a fairly amateur smasher. I don't know too much about anything, to be honest, but thought this would be a good enough suggestion to spitball around. Hopefully somebody who's good enough here takes a shine to it.
Movement
With X, I always imagined he'd have a running speed somewhere close to Wario's. Not incredibly fast, but not incredibly slow, either. X had a sort of slower run in his series, anyway, so I don't see how that'd detract from his character. As for his weight, I felt matching it close to Link's and Snake's would be a nice fit for him, with roughly the same falling speed. Height-wise, he'd probably be as tall as Ike, but it's likely that he'd be kind of bulkier, like Ganondorf. I imagine that his traction would be on par with Cap. Falcon's.
X would be able to wall jump, of course, and could possibly have his neat little slide from his series. I'm not sure how that would hurt his character at all.
Combat
Normals (ground)
Jab: A solid one-two punch that ended with with a 'shoryuken'-esque uppercut, but without the huge jump height or anything crazy like that. The trajectory on the last hit of the jab could probably be very slightly upwards and away from X in the direction he's facing. I always imagined that the first hit would do 4%, the second would do 3%, and the last would do 3% as well. The speed of the punches would be similar to the speed of Ike's jab.
F-Tilt: A quick thrust forward with both arms, pushing the opponent back. Damage-wise, would be around 5%. The speed would be comparable to Ganondor's F-Tilt; just a sweet little 'pop'.
U-Tilt: X would do a handstand and thrust his legs upward, lifting the opponent. Would do around 6% damage. I always loved to imagine that there'd be a small sweetspot on the bottoms of his boots (where the little 'boosters' are) that would do about 8-9% damage, and have higher knockback. The trajectory, sweetspotted or not, would be straight up. Speed would be comparable to ROB's U-Tilt, but slower.
D-Tilt: X does a quick sweeping kick with one leg, with two possible trajectories.
a) If hit at the tip of his leg (calf to foot), the opponent would be sent straight up. 8% damage.
b) If hit with the upper part of his leg (thigh to knee), the opponent would be knocked back and away from X. 6% damage.
Smashes
F-Smash: X quickly trusts a leg out in front of him while firing the booster on his boot, sending the opponent straight back on a horizontal trajectory. There would be a hitbox for his foot, and for the most of the rest of his leg (about to his thigh). The upper hitbox (about to his ankle) wouldn't send the opponent back too far, and would do about 9% damage. The tipped attack with his small thruster would send the opponent back farther, and do about 11-12%.
U-Smash: An actual 'shoryuken'-esque uppercut, complete with a really short hop into the air. The attack would sort of 'catch' the opponent while lifting them, doling out three hits total, with the final hit being the strongest. Trajectory-wise would be up and away from X in the direction he was facing when he performed the attack. Damages for the hits would probably be (uncharged) 3-5-6 to total out at 14%.
D-Smash: A slower version of his D-Tilt, which hits both front and back of X, with two trajectories for each side (which are the same).
a) if hit at the tip (calf to boot), the opponent would be sent upwards and away, with about 10% damage.
b) if hit from his thigh to knee-ish area, the opponent would be 'swept' inward towards X, at about the same amount of damage).
Aerials
N-Air: X Does a quick somersault, sending the opponent up and away in the direction X was facing. About 6% damage, with higher knockback at the start of the attack.
U-Air: X's helmet flashes, and he does a quick headbutt, sort of like how he uses the head upgrade in X1 to bust through blocks. If the opponent is slightly behind X's head when he does this, they'll be sent upwards and backwards, where if they're hit where his face is, they'll be sent forwards and down at an angle. I don't hear of or see too many U-Airs that perform a function to send somebody down, so I figured it would be a sort of unique thing for X to have. Damage-wise, would probably be about 6% for the behind hit, and 7% for the forward hit.
F-Air: X would do a sort of 'drill' kick with a short duration, working similarly to Mario's D-Air in 'holding' an opponent in place. Its knockback wouldn't be too noticeable since you're caught in the attack for the duration, but I figured it would sort of 'drag' you towards X. There would be four or five weak, rapid hits, each dealing about 1-2% damage. It would be an option versus shields, was my reasoning for this at first.
B-Air: A backflip, in which X brings his leg down on an opponent from above, sending them downwards at an angle. If the very tip of his foot connects, the opponent is sent straight down. The attack would do around 6% damage.
D-Air: X does a sort of 'punt', like he was trying to kick a ball off of the ground. The 'swing' of his leg would send the opponent forward and slightly upward if not tipped with his boot, and would send more upward if tipped. The attack would do about 7%.
Specials
Uniquely, something I wanted with X, was for the ability for him to apply charges to some of his specials, as he can in the X series. I don't know what everybody's take on this would be, as it feels like it would simply give him too many options from attacks, but I'll just post the uncharged and charged versions. Most of X's specials would have a little bit of endlag after them, where his neutral wouldn't.
Neutral: X fires his X-Buster (kind of a given, really), which would fly at a slightly slower speed than Falco's lasers, but would have a lower 'cooldown' between shots. Like in the X series, his buster wouldn't cause opponents to be stunned, which would (I hope) balance out the speed and range at which he can fire them. Uncharged blasts would do about 2% damage. X would be able to move around freely while firing his Buster, and could of course charge it. The kicker though, is that being hit while he's charging will knock him out of the charge, and require him to start again (assuming he gets hit by a move that makes him flinch). Being able to move around while charging also means that he cannot perform a dodge roll out of it like Samus, DK, Sheik, and other characters can take advantage of during their chargeable neutral specials.
A charged blast from his neutral special would fly slightly faster, be slightly larger than the lemons from the normal shots, and apply flinching as well. The attack wouldn't really have knockback when charged, but would simply have high hitstun. A full charge could be attained in about 2-3 seconds (I feel like that's a long time in Smash Bros, anyway).
Forward:
X uses his trademark dashing ability to quickly close distance on an opponent. The dashing distance wouldn't be too long, but he'd be able to jump out of it to allow for some longer jumps and aerial approaches from a distance. In the air, X could use this to approach the stage and aid in recovery, as he would not fall during the first one used in the air. After the first, X will still get the horizontal distance in the air, but will fall like normal.
Up:
X's Storm Tornado ability, obtained from Storm Eagle (I understand that, in the actual X game, the attack is horizontal, but, hey, it's all I could think of for recovery). X would barrel through the air, 'carrying' those he connected the attack with along with him. Though the attack lacks horizontal reach, it would be a very 'tall' attack that covered him from above, and could prove to be a strong finishing attack. X would not fly to the top of the tornado, which itself would be about two-and-a-half times his height. When charged, the attack would launch horizontally in the direction X was facing (like it normally does uncharged in his series), acting as an attack that could carry opponents off the side of the screen. A full charge would take about three seconds to ready, and could be interrupted like his neutral charge.
Down:
This is a weird one, so bear with me here. X remains in place to build up a charge, needing about six seconds total time to do so. The charge can be interrupted with a roll like Sheik, DK, Samus, etc, but is lost if hit while charging. When fully-charged, X temporarily gains his Body armor piece, and greatly increases his knockback resistance for about seven seconds, or sixty damage is accumulated; whichever comes first. It's a defensive tool that allows him to take greater risks in his offensive game, and could potentially be very rewarding if the time is taken for a charge to be completed. I'm hoping that this one won't be too powerful, what with an opponent respawning on the stage with an X just waiting there and charging, ready to tank out an attack again. I really had nothing else to think of for his Down special, but that struck me as something unique.
Again, sorry if this isn't too polished or anything like that. I'm by no stretch of the imagination a high-skill player, but I'd had this idea bouncing around in my head for years and years now, and felt it appropriate to throw it into the board for some healthy discussion.