Notably, there have been two Lilah/Sarks, both pretty good. And a Lilah/Lauren.
But yes, Lilah fits in *so* well with the Spyfamily; it's not hard to imagine Lilah as Irina's lawyer or one of her proteges. It's pretty much a natural crossover.
Hmm, what else? I'd like to see more Kill Bill crossovers. You know that Bill would take to Lilah. And then there is Sofie Fatale or Elle, both of whom work well with Lilah. *grin* I actually have a workable HP crossover, but it's too involved to be interesting to anyone outside of me. I'm also amenable to an X-Files crossover; Krycek teaches Ms. Morgan a lot of lessons in her misspent youth, for one.
Lilah can do a lot of crossovers: pop, Alias, X-Files, Kill Bill, 24; I think there's a workable Sandman crossover; given that SR did an ep of Due South, it would be fun for there to be a Due South crossover, same going for The Sentinel. Hmm; definitely the cop shows...Homicide, ER, L&O. There's a possible L Word crossover, maybe Six Feet Under...I'm working on an Arrested Development crossover. I liked Fox's Everwood crossover.
I mean, the nice thing about Lilah is that everyone eventually can use a lawyer, and she has a large amount of unaccounted time to play with, either as Delilah corporate lawyer ho, or young player, or even just as confident and vulnerable woman looking for love/sex/adventure. So short of history or fantasy fandoms (I think Trek, LoTR, even HP are sort of pushing it), Lilah can cross over quite nicely. Which is nifty-keen.