Very minor concept for SNES Super Mario All-Stars (and the SMW combo cartridge):
The Battle Mode found within Super Mario Bros. 3's title screen is very engaging and compelling to play, but the ? Mushroom item that can be found in certain blocks throws a wrench in all of that under certain conditions. If both players are Super or both players are small, it has the effect of switching the players' location, which is cool and good. However, if one player is Super and the other is small, it also has the effect of switching their sizes, which is dumb and bad. It basically forces the player with a size advantage to purposefully get hit in order to prevent the opponent from gaining a significant advantage that was only possible because they were playing poorly to begin with. I imagine the item does the size switch mechanic regardless, but switching Super with Super or small with small doesn't actually do anything noticeable.
I think the ideal way to fix this would be to make the ? Mushroom only switch player locations regardless of the players' sizes when it's grabbed, disabling the size switch mechanic. It gives both players an incentive to go for the item in a mismatched situation, rather than just the losing player. The positional advantage is fairly strong by itself for either player in that case, and it still gives the disadvantaged player a bit of an edge (they're more likely to need it in order to get to the top against the bigger opponent).
In case it would be helpful, per SMW Central's RAM map for All-Stars:
$7E18BB is 13 bytes long and refers to sprite numbers. $11 is "Red ? Mushroom". Interestingly, $12 is "Green ? Mushroom", which goes unused as far as I'm aware, and I have no idea if it has a different effect from its red counterpart; it doesn't seem to have any documentation elsewhere. If it turns out that the Green ? Mushroom only switches positions, then simply replacing all Red ? Mushrooms with Green ? Mushrooms would work.
$7E19C0 is a 1-byte timer that is active while the ? Mushroom has been grabbed and is doing its effect(s).