Guinan was meant to appear in the second season episode 'Rivals' with her son but Whoopi Goldberg was unavailable
Also there were rumours Scotty was going to appear, and they wanted Leonard Nimoy to play Mirror Spock once more.
On Yahoo! Directory about 15 years ago (I have a memory for these things!!) there was an alleged unused script for a Picard episode, I can't remember the premise!
Sita Jaxa and Thomas Riker were also mooted for a return (Source : http://en.memory-alpha.org/wiki/Unde..._Nine_episodes
Deep Space Nine through the years was visited by a lot of familiar faces. We could have seen some more.
In USTV, crossovers have become a commonplace, for example the CSI and Law and Order franchise as well as other show like Murder She Wrote/Magnum PI and even 2.5 Men/CSI. A common practice is a two-episode story with each show having either Part 1 or 2.
It's a pity TNG/DS9 didn't take advantage of this even though we had the comics and Birthright I, but Trek only ever crossed over 1 or 2 characters at a time. The shows were all shot at Paramount. The closest we've had is the fan film 'Operation Beta Shield' with the crews of Hidden Frontier and Intrepid, which is ironic as their respective bases are Los Angeles, CA and Dundee, Scotland! Divided by the Atlantic AND most of America!