I would suggest a jump`n`run / platformer game, with
an open levelstructure
(levels can be compiled to
"level-campaigns". The Player can choose the campaign (level-bundle) at startup, and try to archive the objective (time, points, limited lives)

with a core-team that develops the core-architecture
(logic of the platformer) and an editor or specific (WED)
level specifications.

Then others (artits, newbees) can edit and submit their
own levels to the game, that work with the provided mechanics.

This way experienced users (program/mechanics), artits (levelspecific props) and newbees (level-editing)
can work together.

The game can then grow with the new leves provided.