I don't think involuntary random encounters would really work well with the style of the game
That's actually the easy part.
All the dungeons and towns are generated during worldgen, they're just hidden until you reveal them somehow. So we can simply check if there is any dungeon on the worldmap tile before generating a wilderness location.
So what do we do if they try to descend on wilderness that they just haven't revealed yet? I guess that also depends on whether we want to save each wilderness area so that they can return to it. If so then that "wander around some musty ruins" idea would probably make most sense. If we just dispose the wilderness areas then it doesn't really matter