Once you've been going for a while, you may come up with monsters or other antagonists interesting enough to make a return appearance. So take a look at the monsters that became recurring threats or out-and-out Big Bads, and see what can be learned from their examples.
The Daleks got in early. There was no easy way for the belligerent cavemen from the one previous serial to start turning up with any regularity. So who were your earliest monsters? Of course, they kept coming back because they were interesting, and you could do a lot with them. They represented the spectre of nuclear war, and fascism to boot. They were characterised individually enough to allow for power struggles, dynastic conflicts and coups. They had spaceships and before long time travel. They were easy to draw, had catchphrases and voices you could imitate on the playground, and made cool toys.
The next breakout monsters, the Cybermen, had great thematic weight too, as well as the creepiness of techno-zombies and, by their second appearance, an iconic look. The Master was designed for multiple plots, being both a "hero's shadow" antagonist and a pulp criminal mastermind. Davros gave the Doctor someone to talk to more directly in Dalek stories.
So look at your creations. Who was a hit with the players? Who do you have more ideas for? What might you change, since the adventure could be centuries apart for them? Whose themes and metaphors resonate with you and your audience?
For me, for example, I brought the Tarokka back as guest stars in the series finale of The Door In Time S1 because the players had enjoyed tangling with them, they had a cool (borrowed) visual style, and the central theme of beings so focused on an ongoing war that they don't care about anything else has definite mileage. (The Sontarans have it with the Rutans, but the Sontarans have plenty of other material to work with.) So they might reappear in S3, and might have already reappeared in S2, still fighting their endless, essentially pointless war of attrition with the Vas Murb.
Example: Masks Of The Galatea
The Doctor, Emily and Richard visit the 28th Century and finds that the Galatea, vain cybernetic raiders, have made an uneasy peace with the human residents of a system of colony worlds... but two rivals for leadership of the Galatea crew threaten the truce and could bring the whole solar system to a chaotic war in their greed and petty rivalry.