Welcome. I'm typing this from my frankendox, which was made pretty much for the same purpose as what you mentioned.
I primarily play Dota 2, Rift, random FPS, ARPG, RPG, and racers on my couch and my desk using similar setups, so I guess I have most of it covered?
First thing to consider would be movement and control keys. For me, having access to WASD and/or QWER, 1-5, the first few function buttons, ESC, and SPACE is enough to play most FPS, MOBAs (control only), and modern RPGs. For this, myself and dorkvader (truly, my inspiration) use a single handed Ergodox (left side) with the standard wiring except for the flipped teensy and a different firmware (soarer's).
I guess, if your layout can emulate a full keyboard then you should be home free. I manage this by adding 2 layers, one for function keys and another for the numpad.
For MOBAs, I primarily communicate using text chat (I find voice obnoxious), so the right side ergodox fits this perfectly well. I also have some chat macros binded in for most situations, so that covers my primary game.
As for the cost, prototyping a PCB seems to the biggest cost driver for a truly customized MKB. For the case, a cheap acrylic case will serve you very well, and teensy controllers (and their cheap knockoffs) aren't hard to find. I guess you can wrap up a TKL in less than USD 100 (maybe a lot less, please correct me if I'm wrong) if you're so inclined.
Switches... well, different strokes for different folks. I can't say for sure which switch will suit you well, because I got to my favourite switch in a huge roundabout fashion. I guess you should try out all the switches before you commit to one, but making a custom plate with cutouts to remove the covers will make it safe for you to try out different switch types with a small marginal increase in cost.
Honestly, I'm not that experienced with custom boards yet, so I might be wrong, but the gist of it is there.
