I don't really get what you mean. Can you give an example? Do you want the game engine to unify the interface over many different games so that you can switch from one game to another with less work?

I'd like users to be able to customize the controls and hide options that make no sense for the function of the game they're playing. Perhaps a scriptable user control interface that allows plugins for different game types would be a more optimal solution. Granted, I haven't messed with Vassal in a while, but the last time I used it, it was just a mess that would take longer to explain how to use than it would to explain the rules of a brand new game.

Our experience has been that the two things you're asking for---a shallow learning curve for learning to use a game's UI and a UI customized for each game---aren't compatible in practice. We arguably have the latter, a UI customized for each game, right now and it generates no end of confusion because users expect the UI not to change completely between game X and game Y. If you give the module designer broad freedom to alter the UI, then what you get are differences in game UI between games which are down to the designers being different people more than differences between the games themselves.