I've been following tor.com, where Keith DeCandido has just been rewatching Rejoined, an episode of Star Trek: Deep Space 9. This is the one where Dax meets someone from her past - in fact, an ex-wife of one of the symbiote's previous hosts (writing about Trill relationships can get complicated). In fact, the other Trill has a new host, too, and she is played by Susanna Thompson, who is playing Moira Queen in Arrow at the moment, Oliver's morally dubious mother.
The episode features the first kiss between two women in all of Star Trek, and it was quite a controversial scene at the time. Keith DeCandido points out that, in episodes where a regular character meets an old flame, the old flame has only a short time to establish themselves and convince the viewer that they really did share a passion - and he says "this time round they struck gold."
I was also interested to find that this isn't the first time she's played a Queen, as she also played the Borg Queen in several Voyager episodes.