So, this idea came about in the custom moves thread. The idea is essentially that some custom moves based off of canon abilities could be found in adventure stage maps. For example, find Samus' Plasma Beam in the Metroid level. Now, most moves likely would not be acquired this way, and not every series may get a stage. However, in cases like that, let's say that Mega Man can find copy data (in the form of the drops from the bosses in MM1) in other stages. There could even be sections of a Zelda stage where Link can only get one move with the help of another move (automatically equipped upon finding them).
Now, there are some problems such as it taking more resources to make extra areas and potentially making a simple adventure mode too long, but I think it would add to an adventure mode and the ill-received custom moves.
Now, there are some problems such as it taking more resources to make extra areas and potentially making a simple adventure mode too long, but I think it would add to an adventure mode and the ill-received custom moves.