its always pretty hard to give that kind of advice as each person has their own fav focal lengths, but here's a thought: get a 15/4.5 Heliar. a lens this wide takes a few rolls to get the hand of it, but once you do it's kind of addictive, an ultrawide can be really handy in very tight spaces or when you want to capture the whole environment. an LTM 15mm with M adapter will be around $500 used, it's really tiny and it's far enough from both 35mm and 50mm.