Personally, if I had to choose between which Mega Man incarnation to include in Smash 4, It'd be a tossup between X and Volnutt for me.
Why those two?
Well, let's sit back and consider why I'd say Volnutt. Yes, it's partially because I'm a Legends fanboy, and the guy needs some exposure. Has he been on a Nintendo platform? Yes, he had a port of Legends on the N64, aptly named Mega Man 64. Not only that, Legends 3 was SUPPOSED to be released on the 3DS. Would that be good enough to get him in? Who knows; Snake had a port of MGS on the Gameboy Color, and The Twin Snakes on the Gamecube, not to mention the large number of Metal Gear games on the NES, though those are oft-forgotten in comparison to the Metal Gear Solid titles.
Volnutt also had a taste of the Fighting Game scene in Tatsunoko Vs. Capcom, as well, where his playstyle involved swapping his sub weapons which game him a large variety of specials, finishers and the like. This brings me to my next subject of discussion; Volnutt's moveset.
It'd come as no surprise that his moves would mainly consist of punching and kicking. However, he wouldn't be the kind of character that'd fall under the "Mash A and win" flag. His moves would focus more on pushing back or stunning the opponent, with a strong focus on kicking, as his legs have a far greater reach than his arms do.
His Special Moves would center around the use of his various Sub Weapons, though his Buster Gun would take up the regular B button slot, with the classic 3-shot burst most Mega Men are known for. Since Volnutt's Buster Gun, unlike most of the other series protagonists, is incapable of charging, the individual shots would have to provide a decent amount of punch and knockback, similar to Falco's blaster.
His Smash B would be him rushing forward with his Drill Arm for a set distance, and then stopping to spin the drill itself, which would spin indefinitely until the B button is released. The main objective, when using this move, would be to rush forward, and catch the opponent in the deeper part of the drill's hitbox, as the drill would push the opponent forward, out of the hitbox.
His Down B would be the setting of his Splash Mines, which would work on a timer or contact, and detonate within a small radius the mine itself designed to knock the opponent on their back, allowing the player to either close in for a combo, or make a quick getaway.
His Up B/Recovery gave me a bit of trouble, as Volnutt has no helper animal that can allow him to reach new heights, like Mega Man and Rush. Though Data is his companion, his primary function is (natch) data backup. (That, or being a punching bag, which I'm sure many a player wished they could do to the little monkey sometimes.) So, my first thought was to incorporate Data into the Up B by summoning the little furball, functioning like Mega Man's Mega Ball from MM8 when jumped upon, or acting like a Koopa Shell with limited range when kicked or picked up and thrown.
His Final Smash, assuming they return in Smash 4, would be him equipping the Shining Laser, staying in one place, and firing away with impunity for a set amount of time, causing massive amounts of damage to any poor sap caught in the beam. Thankfully, it can't penetrate walls or platforms, so the best defense against this Final Smash would be to find a wall or platform and hope that the laser can't reach.
==========
On the other end of the spectrum, we have Mega Man X.
He got his start on the Super Nintendo Entertainment System, where he went through three games before he migrated to the Playstation with Mega Man X4. He also had two games on the Gameboy Color; Mega Man Xtreme, and Xtreme 2.
A few years later, he had two cross-platform releases on the Playstation 2 and Gamecube with Mega Man X Command Mission and the Mega Man X Collection.
In contrast to Volnutt and his older brother, Mega Man, X himself has had no playable appearances in any Capcom produced or co-produced games, though he has had his fair share of cameos.
Moving on to X's moves, (heh) he would most likely adhere to the classic "punch punch kick uppercut" combo. He'd be speedier than Volnutt, but consequently, would not be able to pack as much of a punch. (heh heh, two for two.)
For his Special Moves, he would use Special Weapons from Mega Man X4. His Standard B move would, of course, be his X Buster, which you could charge. Unlike Samus, you could move while doing so, though the fully charged shot would be smaller and faster than one of Samus' fully charged shots. His mid-charged shot would be, of course, half as strong, but would have an electrifying property to it. Of course, you could forego charging, and spam the lemons, of which there could only be 3 of each of those shots on screen at a time.
For his Smash B, he would fire the Double Cyclone, which, like the actual weapon, would move forward, then curve upward after a set distance. Provided a character is light enough, they may be carried by the wind-based weapon, so it'd be safer to use a heavier character if this weapon is shot at you.
For his Down B, the Ground Hunter special weapon would make the most sense. Up to 3 can be on the screen at the same time, and they can travel down walls, but not up, disappearing upon impact with an opponent or a destructible piece of the environment.
His Up B/Recovery move would be him using the charged version of the Rising Fire special weapon. As such, there would be a couple of seconds before X actually uses the move, where he would be vulnerable to attack.
His Final Smash would be him equipping the Ultimate Armor for a set time, which would decrease the charge time of his X Buster, as well as increasing the size of the Fully Charged shot, and giving it the signature three Plasma Orbs that are left behind as it travels through an enemy or destructible object. His Smash and Down B moves would be the Fully Charged versions of the Double Cyclone and Ground Hunter, respectively, and his Up B would be replaced with the Nova Strike, which, not only would it allow him to move up a little ways, but cover a decent distance in a short time. He would gain a third "jump" in this form, which would come in the form of an ability to hover left and right for a short time thanks to his Hover Boots.
Alternatively, we could go with an X1/X2 inspired moveset for his Special Moves.
His Standard B would remain unchanged, however, his Smash B would be the Hadoken, albeit nowhere near as powerful as it would be in the actual Mega Man X game. His Up B would be the Shoryuken, which, like the actual Street Fighter move, and Luigi's Up B, depending on where the opponent is on impact, the opponent may be engulfed in flames, though unlike Luigi's Up B, this will not send the opponent flying upwards.The Down B would be X firing the Rolling Shield special move, which will roll along the ground until it falls off the stage, reaches a wall, or "explodes" upon hitting an opponent, or destructible piece of the environment. His Final Smash would be the Giga Crash, which would involve him moving to the center of the stage, and several shots being fired from the top and bottom of the screen, These move fast, hit hard, and are unimpeded by obstacles, but are small in size. The best way to avoid this attack would be to time your dodge right, or risk using your shield. If you're in the wrong location, chances are, your shield may be broken, and you'll be left open to attack.
