One thing I was doing with my buddy is a big team challenge. We selected 8-player Smash, and made the teams two versus six. We played on a team against six CPU's all set to 1, and when we beat them, we turned all six of them up to 2. We turned the CPU level up each time we one until we couldn't beat a team a six level 5 CPU's against us.
As far as themed matches, you could do a team game of "Hero-Villain Cross-Up". Teams would be Mario and Ganondorf versus Link and Bowser, or swap out Dedede and Kirby. Change it up with variations like "Princess Cross-up", Mario and Zelda versus Link and Peach.
Really, look at the Challenge battles in online mode for themes. Find what characters have in common, and build teams accordingly, just like they do. For instance, Fox, Falco, Captain Falcon and Samus could be "Team Pilots." Link Zelda and Ganondorf would be "Team Triforce". Falco, Pit and Charizard form "Team Wings". Then, get some smashes going! R.O.B. and Mega Man versus Robin and Ganondorf could be "Robots versus Wizards"!