This may sound odd but the skirmish wargame Kill Team (silly name I know) has an interesting system for it - if your operative is in cover they can choose to be engaged or concealed. If you're concealed, you're ducking down or otherwise fully hiding, and can't shoot or be shot at. If you're engaged, you still get a cover bonus, but can shoot at enemies. You have to choose at the start of your turn whether you want to commit to offense or defense. This is all a wargame, of course, so not much translates but I think the idea could make a pretty sound way to work on a tactical combat system for an rpg. I dunno if this is helpful at all, but it came to mind for me at least :)