TPIC definitely. Get it as soon as you can. You mentioned that you're already two years in at a regional. That probably means you have two years SIC jet time anyway. Round your resume out by getting some TPIC asap. Doesn't Colgan/Republic have a two year upgrade on the Q400 now? I'd be all over that if I was trying to build TPIC 121 time.
Oh, and the recommendation about being a check airman if you can........absolutely. Very good advice.