I, personally, would rather that they invest their time otherwise... Whenever I think about the hundreds of stickers and trophies, I can't help but think about how they could have instead worked on another character.
That being said- if they brought back something like stickers, these are the things I'd want to see changed:
1. I kind of would like it to not be something where you actually stick it to the base... I hate negotiating each pixel to get as much sticker space as possible. Just tell me that a sticker takes up 15% of your available space and switch around based on that.
2. Have a mode where you can actually examine a character's moveset and see which tags apply to it, i.e. See if Zelda's side B counts as fire, explosive, magic, or all three and then compare how much damage is based off of the original attack and how much each sticker is adding to it.
3. As you said, you should be able to remove stickers without wasting them.
In regards to the multiplayer mode, if they have a sticker mode it'd be cool to see that as one of the special versus modes. There'd be things to figure out, of course, like whether each player applies stickers separately and stores those in their name.