Looking at all these shows of today, with the CSIs, Law & Orders, and Chicagos series, they do cross-overs all the time amongst their different spinoffs. I know trek was ahead of the times with spinoffs, but im surprised during the first season of DS9, they never did a full-on crossover with the highly popular TNG at the time. I could easily envision a 2 part episode, part 1 being TNG with a few DS9 guest stars, part 2 being DS9 with a few TNG guest stars. It would have given a great exposure to the early DS9.