Any Assets are usable as long as you give credit.
You can use any game engine: Unity, Unreal etc....
and yes TPS is great! I did say get creative didn't I?
I would like you to have at least Windows and Linux, but if you want to make it playable on mac Android, Ios , web browser that's great too!
You do not have to make dedicated server, but at least something like with bots or a way for players to host servers.
I would like to see some realism, but anything will do, also a good gore system is fun.. but just get creative!
Also all type of software is great!