Fix Klaw's hitboxes for sure and think about a better way to deal with Bowsercide. Obviously the system now and the system at launch isn't quite right. I think the best course of action would be to make enemies enter freefall when they are let go of after Bowser dies. This ensures that all characters grabbed can't recover when Bowser dies first, as well as covering up other problems/potential exploits. The Flying Man glitch doesn't stick around, people grabbed can't just jump back to stage (covering up some sad MUs), Bowser can't use a win move when he's on his last stock, etc etc. I honestly think this is the most balanced klaw change we could make and would overall fix all issues as long as the current priority system is left in place.
Bowser needs more than a suicide tool to fix him up into a tournament threat, though. The rest of the changes have been stated in this thread and I'm going to list them off in no particular order.
Firebreath is an insane zoning tool that we can't use properly due to the very large punish window - even when it doesn't whiff. It also decays quickly and is kinda small - Charizards firebreath is much better imo. Some small tweaks to firebreath would really go a long way.
Tough Guy is an inconsistent mess. Rework him so he has constant armor against certain attacks (preferably ranged ones) or make it more concrete so that it isn't affected by rage and current %. Make it something we can keep in mind and take advantage of when an opponent tries to wall us out with weak attacks or ranged nuisances (I'm looking at you, Falco.)
Shellguard on all shell attacks is a questionable move - Grounded Fortress is his best OoS option, deals nice damage and comes out faster than his jab. It's reliable, has plenty of hitboxes and lasts plenty long for punishing dodges and even landing options! Dair is a wonky move in the way it works but it already has plenty of killpower and lots of priority. People often respect dair and punish later, although only a few characters have moves Bowser must respect when it comes to dair. For some reason I can't fathom a buff to dair.
Finally, Dsmash. Basically, a stronger version of Fortress that doesn't move but has more range and killpower. Hitboxes stop coming out sooner resulting in a higher endlag and is overall considered one of his worst moves. I've gotten some disgusting edgeguards with it and if you can read a dodge or landing option it's a reliable kill move - even moreso after the buff it got. However, it's easily punishable and still isn't that great. Drag out the hitboxes so that you're dealing damage for longer, or add more hitboxes and decrease each one's damage so that it has more active frames. Furthermore, a windbox would be helpful to pull in enemies that try to stand just outside range. Finally, concerning shellguard: I could see this move having it. Fortress already moves and has more active frames - perhaps trading maneuverability and active frames for damage and armor wouldn't be a bad idea, giving us options for different situations.
Uthrow being a more consistent combo throw would be nice. Bowser already has great grab range (sweet jesus that pivot grab), so a change like this would be nice. Decrease the damage of uthrow to, say, 4-6%? At 0% it might combo into nair but after that it should be a DI choice of whether you need to fair or uair for the combo.
Dthrow forcing a tech chase situation would also be cool, as it would allow for more mindgames concerning "is he going for the combo or do I need to choose a get up option?" These changes would give Bowser an unparalleled throw game (Since we already have Bthrow the kill move when we have suitable rage (rip Ness)) which can be considered overpowered and we'd have to pick and choose buffs of this caliber.
Fixing jabs so that Jab 1 leads to jab 2 every time sounds nice, same goes for fixing the rising hit of Dspecial. Making it so that fortress doesn't spit out enemies at higher % before the final hitbox actually launches them would be nice - I can't count the number of times they fly across the stage in a trip animation during fortress, robbing me of valuable % as well as positioning advantage (ESPECIALLY on zoners!).
Anything else I should add to this list? I don't want to be missing a single reasonable point or idea.