They did, but I doubt they'll do a second, simply because it's one of those ideas that will only work once.

Possibilities for District 10: alien uprising, Christopher returns with more of his race and kills everything/takes everyone away, Wickus is returned to human form.

* An alien uprising would turn it into a generic gunfight.
* Prawns killing everyone would turn it into a generic gunfight.
* Prawns going home... wow... five minutes of special effects and then... ?
* Wickus being returned to human form is, at best, a sub-plot.

Yes, they've left it open, but in a way that makes you want to know the answers, rather than in a way that would make a new film worth it. At least, that's what I think.

A lot of the best standalone films show you a snapshot of the whole - you don't see the absolute beginnings of the events that lead up to the snapshot, and you don't see anything after the snapshot. Rather, you watch the snapshot (which is always the two or three days in which something happens after not happening for an arbitrary number of years) and are left with questions both about why it happened, and what happens next. I'd personally be very disappointed if District 10 was made.