What about a "choose a theme" challenge, where you can... well, choose a theme from the options given to you. Yes, all challenges have a theme, but this would include smaller themes which are too small for standalone challenges.
Examples:
-HGSS Sinnoh evolution run (The 5 new dex additions)
-Signature move pokemon run
...and that's all I have.
And then the "Weak pokemon challenge", where you can only use pokemon which have a Base Stat Total of under 400.
