Future expansions.

There are already glories and stream spirits drawn using the same template.

Yes, but they have different abilities. What can we give manticores? A poison attack?

And manticores, it seems, have already been approved as part of the faction, plus they really suit the dark elves...

I haven't heard that... Although if it's mentioned in the artbook, then okay, or if it's posted somewhere on the official website, please show it.
Yes, and you know what, even if they come with a gag, I want hydras, period.