Oh, that's a topic I seem to have missed out on. Lets see what I can come up with:
JAG: While I like that show, I cannot imagine a smoothly working crossover with LnC.
Stargate SG-1: Same here, although some kind of investigation in an extra terrestrian on earth by SG-1 would be cool. Haven't seen that yet.
Smallville: Well, that would be my favourite. Grown up Lois and/or Clark visit an alternative universe where they meet up with Teenage-Clark, -Lana, -Chloe etc. I'd so be looking forward to Lois' thoughts on the ongoing teenage romance between Clark and Lana, lol.
There sure are some other shows which would fit but those I never watched regularly.
