Bowser: The moves I would cut noticeable landing lag on are Uair, Side-B, Neutral-b, and Up-B. With uair I would have it kill better at the start of the move, but if you hit with the backside of it there would be a sweetspot that doesn't have much knockback and combos instead. On the ground this could combo into a sped up Usmash(I'll get to it later) at lower percents or even a grab, at higher percents it would most likely give Bowser a fair or bair. Neutral-B canceling on landing could give Bowser some nice grab setups as well.
For fortress, along with the lag cancel on landing I would let him be able to fast fall it after a certain amount of time. Not soon enough he could repeat it at DJC speed though. To compliment this, I'd also make it so he doesn't go into freefall after it. He could slide off edges and platforms, maybe even combo out of fortress if he lands. Another thing I would do is increase the hitbox slightly and have the center of the move be a sweetspot that kills off the top at decent percentage.
Utilt could be a little faster, as the sped up Utilt covers his blind spots well, but not too fast. What I would do is speed up Usmash instead. While Utilt combos better, Usmash comes out faster and has cool down. Another thing I would do is make the ground hit that Bowser does when he lands for Usmash have a pitfall effect. On-stage, this could be trouble for shielding opponents, and off-stage it could spike. Both these moves help him against characters with aerial momentum. Dtilt could also be sped up to help him kill. I kinda want Ftilt to stun people if it hits straight in the face, but I don't think brawl has any headshot detectors. =P
Instead of cutting the lag on dair, I'd speed up the entire move. Fast enough to autocancel and get another aerial off afterwards. Fair could use a slight damage buff and send at a lower angle. Other moves that could be deserving of more damage would be his jabs and grab pummels. His Fthrow could plop people upwards like his Side-B used to do in Melee.
Really I didn't like the thick skin code. At first I thought it was a good idea, but one of my friends is a Bowser main and as I watched more and more replays I realized "Why are all my combos on Bowser so long?" The thick skin code only helps him get combo'd longer, I'd rather up his survivability with better recovery/weight.