Future expansions.

Theoretically, the swamp could be linked to the beastmen, of course (those created by mages). The question is that there are almost no plot developments for them, while there is something to add to the story for the other factions, so in my opinion, they are a higher priority.

Those created by mages – it's a slightly flawed idea. You can add a couple of units, mentioning that they were created by them (with a fantastic story about why and how, and that they didn't stay with their creators). Only a couple, and even then, because of the shortage of units.
A faction specializing in weakening and poisoning.
In my opinion, the Basilisk and Wyverns are their hallmark.
Regarding plot developments, let them brainstorm so that in the next parts, they or other developers will figure out what to do with the swamp, which was so beautifully depicted in the previous series.
And in general, you can quite easily fit the swamp dwellers in between such giants as the Undead, Humans, and Demons.
The main focus should be on the problems with Sanctuary, so to speak, the confrontation between the dirty and the clean