Collectible Card Game Headquarters

Are there any UI improvements in store still or are those developers gone forever?

Yes there are UI improvements planed, and no, I don't believe those developers are gone forever.
Sadly a project about the recreation of the client with JavaFX seems to do no progess in the last weeks.

What are in your opinion the most important things of the UI that have to be improved?
(beside using the existing space for displaying cards during the draft, that's already defined as an issue ).

You should take the MTGO as an example about how the UI should be, and by take an example I don't mean copy it, because I don't want you to get in trouble with Wizards.

There something nice about MTGO (again, the beta version, not the old one) UI is that it's really flexible, it uses as much space at it can, resizes cards when it's needed, there is no "blank" spaces like in MAGE (during drafts and even during matches, the space betweek the hand and the lands is huge!).

I was mostly thinking about maximizing the use of available screen space, being able to resize or reposition parts of the battlefield (stack for instance), better end-user control over thumbnail generation and resolution, but also gameplay wise making sure spells targeting the graveyard show what their target is, or support for some basic messaging/status system to be able to ping other people on the server and ask them if they're up for a game, which they could block/ignore with an AFK status.

Just off the top of my head.

Oh and trying to improve as much as possible readability of the deck builder in cardview mode, being able to display more cards maybe, or optimizing how their display is handled so it is a little faster/responsive when browsing through entire sets. A large part of the fun in Magic is building decks so you have the make sure the deck builder side of things is relatively comfortable, easy and convenient to work with and read.

Also a few more avatars would be cool, what's with the lack of hot fantasy girls?