Which characters would you have liked to see interact together in the same episode or movie, whom we did not see interact?
For example, I was disappointed when Q and Odo did not share time together in "Q-Less." I thought that would have been interesting. But I can think of many other examples: Lore and Q, Q and Douwd, Organian and Trelane, etc.
Characters don't have to be from the same series.