Like @MrChronos said before, may exist fanmade digimons after the game is basic done.
For it, I suggest a Event called "Fix the tree", where a digievolution tree branch is showed and players can submit art to "fix" it.
I mean, for "natural" digievolution
Example:
Fix the tree: Tyrannomon!
??? > Tyrannomon - Metal Tyrannomon - RustTyrannomon
>>>>>>>>>>>>> Ex-Tyranomon
>>>>>>>>>>>>> Master Tyrannomon
Submit arts to Tyrannomon "natural" rookie