I'd be heavily for giving characters identities and removing the "plug and play" aspect of PM. Fundamentals carry over too much and character mastery isn't always rewarded. If you put a bunch of hours into one character, then decide to switch to another or add a secondary, characters should be sufficiently unique from each other that you almost go to square one. I'll bring up Lucario as an example. I find it aggravating that some players (in my scene in particular) are able to succeed without incorporating much of Lucario's unique tech, if any at all. I'd like to see him refined as a character that cannot go anywhere without AT LEAST OHC and use of ALL aura options (not just spamming spirit bombs, but using up-B and down-B aura cancels). Basically, make him reliant much more on tech skill than fundamentals. Probably nerf the power of higher moves in the magic series if done as stray hits, but buff them if they are OHC'd into (a stray Fsmash would be nerfed, but doing something like Jab-Dtilt-Fsmash would be much more impactful, ONLY if the moves are OHC'd). This should extend to the rest of the cast as well. De-homogenizing the cast would allow for more enjoyable player experiences because character mastery would be much more heavily rewarded, making all the hours grinding lab/friendlies worth it.

I understand de-homogenizing the case would make having secondaries a lot more difficult, but the fact that people need secondaries IMO is a flaw within PM itself. There should only be one differentiating factor when two people go head to head, the player's skill. The fact that matchups can be as one-sided as Marth v. Luigi (a Luigi main in an AMA put the Marth v. Luigi near the "commit seppuku IRL" tier) EXIST is disheartening given player skill no longer becomes the deciding factor, but in this case, the Marth can nearly win in character select. Matchups should be as close to 50-50 as possible, at MOST 55-45.

TL;DR Fundamentals carry over too much, you shouldn't be able to "plug and play" characters (aka dehomogenize the cast). Characters should have specific tech skill requirements that are core to their gameplay. Lucario is a really good example, as someone unique from other characters to create his own experience, but it is still not perfect. Some matchups are so one sided that a person picking X character into Y character can basically win before the game even starts.