Bowser Jr. is a character who kinda relies on gimmicky clown car movement, but in reality it's not good and neither is Bowser Jr.. Another way to look at it is that he has a few really heavy aerials in his fair/bair that are just too slow/unsafe to be practical moves that stand to gain a lot from less landing lag, and if he could running tilt he'd have one instead of zero safe ways to pressure shields. Other characters being able to escape juggles doesn't bother him because he sucked at it anyway, and he has a good dair and a gimmicky extra option in his up-B so he's relatively helped by the airdodge situation overall for sure. If only the engine changes he's probably still bad but relatively less bad?
Captain Falcon is fast with great tilts and has long range aerials that would be a terror if safer. His recovery stands to benefit more than most from directional airdodges just letting him make some short distance recoveries a whole lot safer. He's without a doubt one of the biggest engine side winners, and you should be scared of that since he was probably underrated in 4 already.
Falco's biggest problem is getting boxed out because for some reason he's a low mobility space animal in 4, but he has some good normals. Being able to use them while moving is a big deal, and his aerials are in the same boat and will just increase his approach vectors with less landing lag which is the single most important thing he needs. He's still bad if his movement speed doesn't get help, but he's relatively without a doubt improved by the new engine.
Greninja is a high mobility character with some kinda big aerials, and he has a lot of non-traditional juggle escapes plus a lot of long range chases to deal with the new juggling environment. He might be under the radar one of the biggest winners from the new engine.
Lucas is a character whose biggest problem is that he's bad at approaching since all of his best neutral options are jabs and tilts and he has the unenviable combo of a bad dash attack and a tether grab. His aerials are all useful but don't really work as approaches; they're all just a little too laggy except nair which is really low range. His combos were effective but mostly lower stuff and not the juggle traps common among the actually high tier characters. In other words, literally everything about the new engine is pro-Lucas and I'd speculate that he's another massive winner here.
Luigi gets basically nothing other than being able to attack out of his skid to a halt animation. That's unbelievably incredible since that more than doubles his effective mobility and makes him one of the biggest winners with that alone even if literally every other new dynamic is anti-Luigi, and honestly they kinda are.
Meta Knight is really interesting here. As a multi-jumper, he's better positioned than most to deal with the new airdodge rules, and while his dash attack/dash grab were already good, being able to ftilt on the go is a nice extra option. If his aerials can be used more safely, that could really game change for him; I don't think he quite has the kit to run away with the benefits like the real sword characters, but it's probably more good than bad here so I'd suspect the engine changes alone would peg him as a slight winner.
Swordfighter loves the idea of being able to use tilts while running and his super laggy but big aerials are really happy about the idea of being able to be used in a non-suicidal way. I expect all Miis to be really retuned to a large extent so this is probably irrelevant, but if Swordfighter holds steady he should be less awful than he was before.
Being able to more safely use fair is going to be a significant buff to G&W, less lag on turtle and dair are going to let him throw out some of his more general purpose good options without getting killed as badly as in 4, and while his dash attack was already good, mobile dtilt is the kind of pure oppression that is going to make some salt (G&W has the dtilt of the gods in 4, and people don't notice because he's bad overall). With his weight and options he was already very hard to juggle whereas his range and aerial mobility will hunt anything down so the new airdodge system is okay with him.
R.O.B. has been covered by DJ3DS pretty well, but I'd like to just iterate how good his tilts are and how nice it will be to be able to use them just a lot more actively.
Roy is a high mobility sword character, basically everything the new engine likes. His ability to benefit will be held back by his awful blade strength distribution since the sourspots will make safe fairs and such still hard, but we know of another buff Ultimate gives this moveset to deal with this problem so I have high hopes for this moveset.
Shulk has been mentioned by others; Monado Speed still exists in Ultimate, and man, running around like Sonic Jr. with actual options this time other than run up and grab and then being able to pivot into Shulk's already incredible nair that now has even less landing lag and is so massive it's going to cover whatever? Yes please.
Wario is low key a pretty terrible character in 4. His high aerial mobility combined with a complete lack of ability to actually approach with his aerials is pretty poor, and Ultimate is addressing that significantly by reducing their lag. His dash attack is well below average, and his dtilt is like G&W's just incredibly good on an otherwise bad character so being able to run around and point at things could really open up his ability to actually do stuff in neutral which would be quite the positive change for him.
Wolf is hard for me to really talk about because in Brawl he was pretty literally just bair: the character, but Wolf's gimmick I think was supposed to be added extra mobility with how his attacks move him so he could probably get pretty tricky combining that with actions out of running. Really though, if his whole gimmick remains spamming bair, the gimmick is better if it has less landing lag and if he gets a real game to go along with it that's only even better.