the only occasion I thought of making a shooter/slasher was when I planned to put the Recast/Detour navmesh pathfinder to my editor, based on the Peragro plugin, what I postponed (but not gave up) after playing a bit with its source because of my new RTS game ideas. but I rather wanted a 3rd person game. basically I don't like this genre to play, but I found it interesting is to develop a good team working AI.

the main reasons what hold me back to really start it:
- the missing mid/high quality graphics assets, and a good effect package.
- a shooter imo is really fun if multiplayer, and I don't know how to make a multiplayer shooter that has a fast response.
- in other engines it would be much easier as they have many helper stuff ready to use in professional quality I never can reach. not only visuals, but networking, physics, animation and AI packages.
- no story idea.

what you plan can be reachable by a tiny dev team, if you determine the target well, and keep limitations. the very free movement system (with special wall climbing) would really make it difficult to create a good enemy AI. even a jump up/down system can make AI things hard. and if only focusing on multiplayer it is difficult to collect a large enough player community. but if the story defines that the player character is a special hero, and enemy is more stupid race, then it can work