Do you want to develop a game or an engine? At some point you will have to use a third party library like Direct X, so is it cheating? No, you relatively have no choice if you care about compatibility. Learning how to develop with third party engines is difficult enough and remember you are just one person, there's huge teams developing engines and one person can not get it to the same level of competition and as a learning example is relatively pointless until you know the basics of how a decent game goes together, if you want a challenge take a crack with Unity..
To get it up to a decent graphical standard, chuck the default shaders in the bin and start from scratch with shader lab. Work on adding some advanced features like sorting out the Post AA in DR, things like SMAA shaders. Dump all the post effects and try implementing DSSDO, pristine mostion blur, add other lacking things like a Voxel based realtime GI.. Build your game, sort out all the challenges in AI, controllers, physics, shadows and performance.. Create your own LOD / Culling system is also a good idea.. Come back in a fair few years when you're done. (That's if you have experience and the knowledge to know how to put it all together)..
There is so much to learn about using engines and adding to them, that alone can take many years to master. Never mind trying to build your own engine.! There just isn't enough time, I own a company with a team of 5 very experienced devs and we still don't have enough time and we certainly aren't trying to re-invent the wheel.
Edited by ShadowKGames, 27 December 2013 - 04:15 PM.