Can I bring people's attention back to Ron_Rancor's storyline idea? I think that it is a very good idea, and it would be a good opportunity for me to introduce a new species I had come up with [as the underground species Ron came up with].
They are called the Samdokons [as "Mudokon" means "Inhabitant of Mudos", so "Samdokon" means "Inhabitant of Samdos", Samdos being this new Landmass]. I hadn't thought of them as being Trogloditic, but now that I think of it, they would suit that quite well.
Their main 'gimmick' is that they have no eyes. Their primary sense is hearing, and for that purpose, they emit a continual stream of ultrasonic pings, in the manner of Bats and Dolphins. This aids them in navigating in the dark tunnels underground, where eyes would be of little use. This difference in senses might cause a problem in gameplay terms, except that their pings are so frequent and their hearing so precise that they can build up almost as detailed a picture of their surroundings as we do with vision. The only real difference is that they see only planes, with no colour, so they would not be able to read text unless it was embossed in some way. So, if we were playing as Rex, the screen would look quite different: things would be monochrome [but in a cool 'night-visiony' way

], and there would be little surface detail. On the other hand, Rex would be able to see clearly in places where Abe and Munch would see nothing, because there is no light. So missions concerning Rex would involve a lot of things like sending Rex into places too dark for Abe or Munch. Hmm... I could really do with Eibu here to think of situations...
Anyway, that can be counted as my second piece of input [anything to stop people having a nonsentient creature as a main character...

].