So I am currently thinking about a game-play type that could be an interesting innovation:

Basically the idea is a FPS that plays in a giant near infinite sinkhole (a bit inspired by the Manga/Anime "Made in Abyss") and which has players progressing downward over a series of server instances. Movement could be enhanced with jet-packs and grappling hooks, but there can be also bridges and spider nets and giant fungi etc. to make the levels more diverse.

So it becomes a bit of a MMO, but without the massive server requirements as each slice (literal level) of the pit has maybe 64 players max or so.

An while there can be PvP etc. the main game-play would be PvE with some sort of spider creatures coming up from the bottom of the pit in waves.

The background is that I am wondering how to design a multiplayer game that suffers less from the empty / bot-only server problem we see in Xonotic/Unvanquished etc. So with the above it should be somewhat fun to play solo, but also in small groups as PvE, while also being able to seamlessly switch to PvP (Team deathmatch like) on more busy servers. Furthermore due to the natural progression in difficulty ("deeper" server instances are password protected, so only more experienced players that progressed down sufficiently have access), it would also somewhat solve some of the match-making issues.

Any further ideas how to refine this? Any major game-play issues I am overlooking that might make this less fun?