Z7 Imp and Unicorn Chomper
As we are nearing Unicorn Chomper's deadline, I wanted to talk about how to make them playable without paying (Z7 Imp as well).
Solution, Add 2 chests, much like the 200 star chest that gives you the crown in that they need 200 stars to open, that give reskins of Z7 Imp and Unicorn Chomper. The reason why they are reskins is because the people who actually bought/grinded for them can feel superior to those who aren't.
Also, regarding the XP problem, the reskins both share a rank with their non-reskin characters, also fixing the max rank problem. But for those people to be superior, make it so the max rank is always a rainbow color for the people that got Uni, and make the max rank have a shiny orange glow or something to indicate that they got Z7. Another thing to note is that the bobble heads would share the same pot/grave like the twin sunflower.