“The Host” was another, and of course introducing the Trill species concept opens up for further things later on. The biggest flaw of that episode is how Beverly didn’t seem to consider a continued relationship after the new host ends up being female. Which is a fine reaction, but for the show pushing boundaries they could have at least had her ponder the idea even if she didn’t act on it. After all, who did she love? In that initial version of the Trill, the host was seemingly not in control, unlike later versions like Dax where the symbiote and host mix to create a shared personality.