How about instead of focusing on where characters will be on tiers, I focus on fixing their balancing issues=???
I'll start first with the Original Twelve from Super Smash Bros 64!
Mario is always around the middle, often higher than the middle. I think he could use more strength and knock back with all of his smash attacks, and each should have some ability to get a kill. I think his fireballs need to stun opponents more, and come out quicker. I also think the FLUDD needs to do (a little) damage and actually do a ton of knock back. Mario also needs to jump higher with all of his jumps, specifically his up B. Mario's up tilt, down tilt, up air, back air, and neutral air needs to be able to combo more. His forward tilt needs to do a bit more damage and quite a bit more knock back (although it shouldn't be able to kill) Mario's down air needs to have more knock back, his forward air needs to spike harder. I think these changes would keep Mario as a constant threat at every aspect, making him the perfect well-rounded character.
Luigi is usually under Mario, and below or around the middle. He needs a better running attack (that or it needs to do more damage), he needs to jump higher, needs faster fireballs, and really needs more speed. He could also use a few more combo-able attacks. His down B also needs to rack up damage like Mario's in SSB64, yet it needs to suck in foe somewhat like a tornado (within like 5 feet in any direction or so), especially if they aren't on the ground. It also should be able to make Luigi rise up if you held B (just like if you rapidly pressed B when using Mario and Luigi's down B in SSB64), thus working as a recovery move as well. I also think he should jump higher with his first two jumps, and that he should have the ability to slowly rise and remain in air like he did in Super Mario Bros 2 (go look it up). All these changes would allow Luigi to remain somewhat well-balanced all around, although lacking strength, but having a tad more speed and a better ability to recover than his brother Mario, as well as a ton of zaniness added.
Donkey Kong was slightly below average in 64 and Melee until he got fixed up nicely by Sakurai in Brawl. However, Donkey Kong could use more powerful tilts. Donkey Kong's down tilt should cover more range, and his forward tilt needs to have more knock back. Donkey Kong has several spikes, but all of them need to spike harder than in Brawl. Donkey Kong's neutral B (super punch) should charger faster, do more damage and kill somewhat earlier, thus turning it truly into a super move that, when you have fully charged. The other major change Donkey Kong needs is recovery. His first two jumps need to jump higher, and his up B needs to rise as well as go sideways, and it could also do more damage. Donkey Kong's forward B also needs to stun more at higher damage, setting up moves like his forward smash and neutral B (super punch) to land a final, devasting blow. These changes would allow Donkey Kong users to strike fear into the hearts of their foes.
Samus went from bottom in 64 to slightly above average in Melee to low tier in Brawl. Samus is fundamentally flawed in Brawl. She isn't heavy, she's slow, and she isn't strong.
Samus needs to be able to jump very high, have multiple jumps that look like her second jump (her down B infinite wouldn't return though), needs to run very quickly, and needs to have floaty DI that can fast fall with proper DI'ing. Samus' smash attacks should all be kill moves (minus her up smash, which should rack up a ton of damage), and her neutral B needs to charge quicker, and kill at low percentages. Some of Samus' Melee strengths should return, such as her ability to combo, although I think unlike in Melee, her tilts need more priority, and should be able to combo a tad more. Her down B bombs needs to cover more range, do slightly more damage, come out quicker and explode quicker, and recover quicker. These changes would make Samus a serious threat in any future smash bros game, specifically in close-ranged combat.
Yoshi has generally been a bottom feeder in smash, although he went up a bit from each game to game. He needs better specials, probably more jumps, more combos, and to be much quicker. All of special attacks need to be more useful, and his up B needs to actually make him jump further. His down B should freeze nearby foes that aren't shielded or dodging, and Yoshi should be invincible during his forward B. However, after 2 hits in it, it breaks, and for the rest of Yoshi's stock after 1 hit it breaks. Yoshi's up B should fire eggs faster, and his neutral B should leave opponents as eggs at higher percentage longer, and allow Yoshi to time out when to land a devastating blow. Yoshi's forward air also needs to become a real full on spike, instead of just a semi-spike. The rest of Yoshi's aerials should come out quicker. These changes would allow Yoshi to be dominant on offense from anywhere on the stage, and change the joke into a smoking fast and deadly character.
Kirby went from towards the top in 64 to near the bottom in Melee to still somewhere on the bottom in Brawl. He needs to have more combos like in 64 (but not as deadly combos), more speed, and his up B and down B need to be more useful. I think his down B should push foes back foes about 10 feet whenever the down B lands from just about anywhere within 25 feet on either side (although not doing damage), and maybe stun the foe for a quick third of a second, preventing them from harming Kirby immediately. These changes would make Kirby more of a threat in any match, and would turn Kirby into a recovery GOD!!!!
Fox has always been higher on the tiers in each smash game. All he needs is his fAir to combo well (like up to 12%) on its' own, and his blaster to have less range and not come out as quickly (these would be done to counter camping). I think his tilts could use some more knock back or speed, specifically his forward tilt. Fox's down B would do much more knock back (it wouldn't kill anybody, but it would give Fox space), and would allow Fox to be the pursuer. With these changes, Fox would remain as powerful and useful as ever, with his A-class speed, combo skills, and overall offense allowing him to remain a "contender."
Pikachu in Brawl had a broken as heck quick attack (up B). If you eliminated the ability to attack out of the quick attack, however, Pikachu isn't that great. That needs to change, along with more useful tilts, and a better forward B. I think Pikachu could use some good short early combos too. I like the fast up B and how Pikachu has little lag frames after it, and I think that should stay (to allow Pikachu to remain a recovery God in exchange for weight and easy KO-ability), but attacking while doing it I think is a little too much. Pikachu's down air also needs to serve as a semi-spike and stun more (specifically at higher damage %), allowing Pikachu to pursue a foe down from the air or off the stage. If Pikachu made those changes Pikachu would be a very balanced recovery God lightning fast speed.
Captain Falcon in 64 and Melee was near the top, but in Brawl he was a joke. The physics hurt him some, and so did the lack of combos, but he just didn't have any strong moves. He needs to be much faster, have faster moves, have combo-able moves, and have his ability to kill early and often more. His recovery skills in Brawl sucked, and in general he has a terrible recovery in smash. To counter that, allow him to jump higher, and give him his down B infinite again like in Melee. His up B also needs to go higher, and probably be less predictable. In smash, Captain Falcon's greatest weakness was his recovery. So, while with these changes Captain Falcon would be good yet predictable at recovering, he would be a beast at every other aspect of game play, and would return to his truly dominate form.
Ness has been towards the bottom in each Smash game. His PK Thunder (up B) as a projectile needs to be faster, when it lands it needs to do more (yes more) when he launches himself into foes, as well as send Ness much farther. His regular jumps need to jump higher, his tilts need to be more useful, his PK Fire (forward B) needs to have more range. His PK Flash (neutral B) needs to come out faster and be a legitimate off the stage kill move that goes farther. On top of that, his down B needs to be able to push back foes when he uses it, as well as recover from projectiles. Ness' smash attacks need to be stronger, and his tilts need to be much more useful. Ness needs more combos with both aerials and tilts. If Ness made these adjustments, he would truly be a PSI GOD!