Just brew what you like.
Milds, cream ales, blondes, wheats, APAs, bitters, ambers, and browns all work well as house ales. Heck, a light bière de garde or saison would work nicely as well.
But it doesn't have to be something typical. After tasting Samuel Smith's Winter Welcome (an old ale/winter warmer), I don't think I'd mind having that on tap year-round.
And really, that's all that matters... what would YOU like to constantly have on tap?