Yea, i was amazed when i added fog and windows lighting how everything looked, it went from boring gray poligons to something very atmospheric, so, you should work a lot on lighting if you want your game to look awesome!
About optimization, you can even abuse it to make light flicker, you just need to find where 0 on X and Z coordinates are, if you go from below zero and back on that coordinates you will see how it turning off and on :) Funny how it became unintended feature!
About avoiding 3D modeling, just don't care about quality much, someone surely will like your models and style, even if you don't step into texturing and other mess that i still afraid of, making models that represent distantly something that you want still will work in every type of project!
I hope one of your next game will feature 3d models that you did yourself! It's important step for everyone and when you do it, you will open another universe where you can create everything you want and make it playable!