Staying with the US civil war makes sense, but when they do go to Europe I would suggest rather than just Waterloo you could base a game around the the 4 days of campaigning, 15th to 18th June(maybe even on the 19th?) - Ligny, Quatre Bras, Wavre and Waterloo, it would suit the campaign style of UG far better than just Waterloo on the 18th. Indeed, it's as if UG was made for just such a campaign. I would suggest the Peninsula war too. Some great battles there. The armies were always fairly even and Wellington always chose good, interesting ground.
BTW, it's a great game.