There's many things I'd want to change, but one of the things I hate the most?
REMOVE Rage Effect!
-It helps any character kill much faster if they take enough damage, and it surely helps heavies the most, usually.
-It ruins some combo potential (well, it hurts Palutena (default) a lot, especially her kill combo). Since it hurts Palu a lot, there may be a bit of bias there too.
-It rewards (affects) anyone those who take enough damage, whether you're in the lead or not
-It makes Shield Shulk much more threatening than he should be, since a high % Shulk in Shield Mode can kill much easier thanks to rage's influence alone.
-Lucario. Do I need to say more here?
I still don't even understand why it exists, and it's on par with how obnoxious Brawl's tripping was IMO. While tripping, though it's hated, is random, Rage is there for as long as you take enough damage, and it's truly obnoxious. Some may like it, and that's fine, but I personally hate it to no end, and such a mechanic should never exist in Smash.