I would love to create RTS mod for minecraft, but it looks like it would need to mess with rendering (almost no experience in opengl). Anyone have any ideas/experience/(want to make rts mod)?
First, make sure you're serious. Total conversion mods aren't exactly an easy undertaking (heck, it might be easier to start from scratch than try to interpret Minecraft's code).
Secondly, I doubt you'd need to mess with rendering too much. The third-person camera should be easy enough to modify - a few changes to the angle and an alteration of the height, and you've got a top-down camera. Optimization might be a bit more challenging (you
would have to modify the renderer if, say, you didn't want to redraw the entire world when a top-down view only shows a small portion of it), but any computer that can comfortably run Minecraft now would probably be able to run it with a different camera angle, too.
Third, are you
sure you want to build this in the Minecraft engine? A top-down RTS would eliminate almost all the value of vertical space, since you'd be unable to see most of it. An RTS would almost certainly ditch most of the actual content (items, crafting, etc.), the entire GUI, and would require vast amounts of modified code - I'm having a tough time seeing how basing it on Minecraft would be a time-saver. Not to mention that, if built from scratch, and assuming you finish the game someday, you would be freely able to distribute and/or sell the game as you wanted, whereas a Minecraft mod is inherently limited (cannot possibly sell it, cannot play it unless you own Minecraft, and it breaks whenever Minecraft updates).
So, yeah. I'd suggest you learn OpenGL and build your own engine. If you
really want to use the MC engine for some reason, rendering is the least of your issues - the real challenge will be reading, interpreting, modifying, and for the most part, discarding everything Minecraft already includes.