AOE 2 DE probably will not have more civs in the future.

https://www.pcgamesn.com/age-of-empires-2-definitive-edition/new-civs

Most exactly Adam Isgreen said this about that:

How did you decide on the new civs, and how far do you think Age II in particular can spread in terms of bringing new civs in?

"AI: These civs were arrived at because we were wondering ‘what’s left?’. We were trying to find a period in history which still fit the general time scope that Age II occupies. We’ve covered so much of the world now in terms of that period that, after a lot of discussions, we finally fell on to telling the stories of the fallout of the Mongol empire, which is what The Last Khans is all about.

At this point, I think we’re done adding civilisations to Age II. I do not foresee us adding any more civs to the game. I think we’ll add more content, campaigns, new game modes – all those kinds of things we’ll explore, but even our pro players are kind of saying ‘we’re done, we have enough civs at this point to last us forever’.

It’s a lot to hold in your head, and I think that they would love to get comfortable with the game rather than trying to push into even more civs. There comes a saturation point, and I think we’re there."

Also he said that the new content will not be added in hd.