To answer properly, I think a lot of characters could have simple tweak buffs and suddenly be truly scary/front runner.
Do I think there is some secretly underrated character with strong rep would suddenly be top? Not really, that's not how I work/think. I'm happy to be proven wrong by sudden shock and horror, but as of yet there haven't really been many instances of this. I try not to think "definitely this/that" and tend to have semi-accurate ballpark feels on characters I've studied to some extent (i.e. Doc Mario isn't abysmal was something I was happy to press, then Nairo against ESAM happened and it was suddenly 'fact', I was suggesting that Mewtwo was a feasibly serious threat for a while and Aba picking him up was like "well, this is is inevitable" and bang Pound).
So in essence, a lot of middle-ish characters with a few tweaks could suddenly be gods.
Do I think there are some underrated characters yet to have top rep to really sell their strengths to the general populace? Sure, but I contemplate their shortfalls aren't fully known to me or are at least more difficult to overcome than I expect if they haven't happened yet in any observable manner.
If another string of buffs came to Marth, I would be contemplating a feasible top tier. His current strength now is solid. Ike mains are already starting to get upset when people are nonchalantly rating him below Marth. The end is nigh
But this has a lot to do with their design being a standard "top tier design". We know Marth can be top tier. And characters with similar kits can obviously also be be relatively simple to push towards top tier; "hello Mewtwo" (because tails are sometimes effectively swords).
Range, frame data, mobility -> causes fear, hence shield/other defensive options -> base abilities are fine/great at dealing with defensive options -> also naturally does great things out of shield due to range, mobility, frame data.
If Greninja's fair was sub 10 frame start up, it would be hard for me not to consider him a staple top tier-esque design (well, him gaining a horizontal rising aerial option that hits people on the ground, basically). If Lucario's tilts and aerials were slightly easier to poke with like he had in Brawl, he'd probably be etching towards top tier too; but I think Mewtwo was designated to take that niche from him.
But this style of design that is the basis of the strength of Sheik, Zero Suit, Diddy, etc etc are not the only styles that can be top tier but they really need a lot of padding in certain areas to compete (like let's say, throw in some super armor or invincibility [mario], have extremely strong frame advantages hence insane combo games/KO confirms [fox, ryu], or zonezonezone [Rosalina]).
Cloud is this design but noticeably lacks the forward facing pokes (like standardish fairs or down tilts); I'll get around to it at some point but I currently believe Cloud is a lot more honest than given credit for.
Either way I think the character with the most amount of yet to be seen potential: Corrin
Character with only a few tweaks would be making everyone cry indefinitely forever because it would be super-brawl-wario on crack: Wii Fit Trainer.
But Megaman with less lag on his aerials would be demonic as well. Heck the lists could go on indefinitely.