Considering Minions were stated by Yoshi himself to be useless and just to give a goal to casual players, that adds too much content to something that serves no purpose and would then force people to collect minions who never cared for them if the content offered rewards that's fairly decent.

So it would be fun, but it goes against his design philosophy. If they add in an enhanced version of Monstrosity or Panrakton from FFXI (Precedence in FF MMOs) then it would be a pretty fun system.