Right now a user can choose between using a 2D software renderer or third person 3D with OpenGL. However, as i'm starting to think about using OpenGL for some particle and special effects i'll probably ditch the 2D. The static parts of the levels are also done with tiles and tile maps, which I want to get rid of as the focus is heading more in the 3D direction.
Anyways, i've decided to go into research mode and have ordered some books:
3D Math Primer for Graphics and Game Development by Fletcher Dunn
I've come to the conclusion that my math skills are not where they need to be, so this should get me started in the right direction here.
AI Game Engine Programming by Brian Schwab
I've been putting off looking into AI for far too long. This book should help make the monsters and NPCs behave a bit better.
Game Coding Complete, 2nd Ed. by Mike McShaffry
Normally I wouldn't go near a book that tries to teach you everything about game dev, but this one has gotten so many rave reviews that i'm going to give it a chance. I'm a little unhappy that the code in the book uses DirectX (I use linux on my dev machine), but i'm more interested in learning some concepts and filling in some holes in my knowledge than getting copy and paste code examples to use.
I'm thinking about getting a data structures and algorithms book, just to have one, but right now it doesn't really fit in with my budget.
I also picked up the Mega Man Anniversary Collection. 10 classic games with Mega Man goodness. Can't wait until Capcom Classics comes out this fall (Bionic Commando, 1942, etc).