quicker jumpsquats and lower landing lag are gonna give big characters a lot of help against projectiles, since jumping is typically too much of a commitment and heavies can't get anywhere fast with walking powershields. However, I think you'll get more mileage out of projectiles whenever they do hit due to more hitlag in general.
Here's my take on heavyweights in the demo:

: No more kills on a grab sucks but Donkey Kong's new tools look promising. Super Armor Headbutt can be used as a mixup on their landings, and maybe as a psuedo counter against linear recoveries that have a hitbox. Down B is now a sixth aerial that should set up combos on hit and be fairly safe on block due to having no landing lag. Giant Punch offstage is hype since he can B-Air into that, but I can't see his offstage edgeguards being a threat if his opponent recovers low. He can't dip too far offstage with laggy moves since his recovery is so poor. He's also packing a less commital dash attack and giant punch seems to take 4 frames to pull up shield while charging compared to 8. Altogether I'm not worried too much for DK's viability, but his weaknesses should still be easy to exploit.

I'm lowkey nervous about talking Ganon when there's always somebody coming in and linking me to a clip of his N-Air. Shorter jumpsquat helps him get to his less laggy aerials, but his movement stats don't look improved at all so he'll still be easy to wall out with sword swings and projectiles. I'm sure dash dancing is going to help him get more mileage out of his Dtilt, and Wizard's Foot looks way better to throw out now that it's as big of a commitment as flame choke. However, Ganoncide doesn't award the kill to him anymore, and if it's true that players can mash out of it midair then I will not fear this recovery option from him while edgeguarding. Altogether Ganon should have the most exploitable and ineffective recovery in Ultimate and I just can't seem him competing on the same level as most other characters. Dude's in trouble.

No more kills from Uthrow and I haven't seen any player Fortress Boost so his recovery height may have took a hit. His Up B does appear to have a sensible hitbox now so I'll be happy to at least trade with the opponent rather than lose to every aerial in the game. Bowser's run speed is excellent, so I think the move that will get the most mileage out of run canceling is Utilt. It reaches high and has enough disjoint to beat a Cloud D-Air. Probably also sets up for some new combos with the shorter jumpsquat. Usmash is also still invincible and kills super early. Also falling N-Air should be great now. Its four hitboxes are bizarre and difficult to perfect shield. Falling N-Air > B-Air should be a combo/occasional kill confirm on the whole cast now instead of just Sheik since we can get up there a whole 10 frames faster. New Dsmash looks very slow and Dtilt appears to have the old move's FAF so I can't imagine being able to followup on any trip. Bowser's still going to be a grappler thanks to Side B, which is now frame 6 on the ground and air. No more having to time the move to come out just before landing. It lost its lagless feature, but a frame 6 aerial command grab is frankly a broken mixup and he'll kill even earlier with it when there are platforms. Also now that grabs are nerfed across the board, Flying Slam is about as commital as the average dash grab. If Jab1 still confirms into Side B, then he'll enjoy an old kill confirm from before he went mainstream. I'm not convinced Bowser will be a top tier threat in Ultimate, but I would be shocked if he was low-mid and will probably land right next to DK again.

Wow you are nothing like Smash 4 ike on release. I still think his recovery is exploitable but having options is more than most heavyweights can say. I don't think he'll dominate the game but I'm definitely anticipating a lot of pocket Ikes.
Charizard: Rock Smash was dumb and I don't know why players miss it. Charizard doesn't need to justify his shortcomings now that he can switch to two other characters at almost no cost. Still the big guy's getting a ton of mileage off universal changes. Both his recovery moves have some armor and I expect a lot from his B-Air and F-Tilt. Charizard is still not great, but I don't see Squirtle or Ivysaur having an easy time killing, so the switch may often prove vital.

Super Missile looks like it will lead to a grab or air combo at close to mid range depending it hits or gets blocked. That's pretty much the only game changer for Samus that I can see and I doubt she'll find much success unless she's been given more tools. A better bomb, better throws, or better hitboxes on her normals would bring her up to speed, why is this so hard?
And anybody lighter than Samus and Ike I don't generally consider heavy even though I'm aware many characters come numerically close. Wario and ROB just don't exhibit similar qualities and weaknesses, and besides Link, none of them really felt like a heavyweight in practice. I can't see any of these heavyweights breaking high/top tier. But then again my only lock for top tier at this point is Ryu with all these unknowns.