I agree here, there's not enough exploration-oriented stuff in a lot of the modpacks. I would like to see more mods that add rare terrain features, or things to explore or discover. There was one in Mage Quest (Ruins I believe) that is a good start toward what I'm thinking of.
Edit: Actually, you know... it wouldn't be terribly difficult to make such a mod myself. No new mechanics or mobs or anything, just some worldgen. The trouble is (as always) not getting bored with the project an hour or so in...
That said, combat is an extremely difficult thing to balance when mods are involved, because plenty of them nowadays offer armor and weapons (which can be obtained by largely holing up in a safe area until you climb the tech tree the whole way) that render vanilla mobs trivial. So it'd be difficult to design bosses that present a fair challenge to players without downright requiring that they spend X amount of time holed up in their base.
Also, have you tried Departed? It's very heavily adventure oriented. The only thing I don't like about it is the mob spawning in broad daylight.