The best way to deal with this ... is to think of the final model game model as you sculpt, as opposed to considering the two steps as separate. And also, even starting on the lowpoly model *before* finishing the sculpt. The low is what goes in the game, therefore it's the most important.
Using lazy mouse and increasing roll distance will help you make straighter lines in zbrush. As for the sculpt, just keep sculpting! I recommend finding some reference to sculpt from, and over time as you sculpt more and more things you'll get better at coming up with your own ideas.
Hello Dear friends and Artists, I am presenting to you my latest sculpt , the burrower...from the amazing concept art done by Jerad S. Marantz you can check all of his cool works here. https://www.artstation.com/jsmarantz The sculpt was made in Zbrush, later on I optimized it for unreal engine using PBR textures. I have…