Okay, so let's gather our knowledge of what interactions can be had with the MT in Dragonborn.

I can confirm that there is a quest where they are hostile, and that if you have worked with the Dark Brotherhood they will send assassins after you, carrying a writ mentioning that you work with illegal assassins.

Any word on joining them yet?

The only presence they have is what you mentioned. Hlaalu remnants hired them, you kill them, they have cool black Chitin, the end.

So if you wiped them out you don't get to see them?

That's quite disappointing to be honest.

No, you will see them if you do quests for the Raven Rock councillor. You won't be randomly attacked by them if you did not join the Dark Brotherhood though.

Eh, it is what it is. It's nice to see them mentioned and for them to have somewhat unique armour. There's just a lot of sidequests so it's nice to see a mention to them at all.