What about something akin to DOTA? In that game, you play as a single powerful hero with specific abilities and wargear, but you are surrounded by NPC-generated cannon fodder. Shift the perspective from top-down to first-person, and thats pretty much an FPS/RTS hybrid.
The question is; do you want the masses of lesser NPC's to be player-controlled, or just generated automatically (thus leaving you free to go on one-man rampages, which is what all FPS lives on)? My feeling is that a little real-time control would be nice (ie select from a sidebar or menu, depending on what the enemy is spamming, develop counters or nice combo's etc), but not the full-blown tech-trees of a true RTS.
The question is; do you want the masses of lesser NPC's to be player-controlled, or just generated automatically (thus leaving you free to go on one-man rampages, which is what all FPS lives on)? My feeling is that a little real-time control would be nice (ie select from a sidebar or menu, depending on what the enemy is spamming, develop counters or nice combo's etc), but not the full-blown tech-trees of a true RTS.