(originally from my now-retracted gliding suggestion.)
What makes flying unrealistic? It would still be limited to so many tiles (maybe the formula could be twice or three times what said creature can walk), take up vital action slots and can easily come with its own risks or downsides. Perhaps, similar to the Pokemon games, flying would need to use all three action slots (taking off, flying, and landing again) where only the flying action gives creatures immunity to attack. I imagine, if it were me playing, those two things alone would make me think carefully about performing a fly. Also, if you accidentally fly short of your target or there aren't enough tiles to perform a clean landing (let's say 2 tiles are needed) your creature crashes and loses some health and bleeds. In other words I think there are plenty of ways flying could be implemented without feeling cheaty. Just my two cents though.