You're free to play anything you want after 2. e3.
d4 Nf6 when someone doesn't play 2. c4

against 2.Nf3 or 2.Bf4 i still play e6...aiming for c5 in the near future...it usually ends up with an open c file and a half open e file which has given me good online results☺...should he capture on c5 you recapture with the f8 bishop...if he plays d5 you play cxd5 followed by d6 which gives you a benoni setup...after that g6 with Bg7 and 0-0 usually works well as the g7 bishop is a beast on that diagonal in the benoni☺
i play Nf6 in reply to 1. d4 and it works really well. however some times i see sometimes someone doesn't play 2. c4 and play's something else. is 2...e6 playable after a move like 2. e3 (for example) or do you have to stick with a 2...g6 which i know works against some different moves.