For shooters you could try GZDoom. It started as a Doom sourceport, but at this point it's basically an indie game engine.
There have been a few commercial releases based on it, such as Hedon, Brutal Fate (upcoming), Selaco and for example the total conversion TOTAL CHAOS (though that one is free) that show what can be done even with that 2.5D stuff.
The good thing is you don't necessarily need 3D skills depending on what you're doing.
The bad thing is if you don't, you need 2D spriting skills.
Also, GZDoom has a really shitty netcode, so if you plan on doing anything multiplayer focused, you will have a bad time. It's really time for Zandronum to upgrade.