First want to say that for an Expansion DA exceeded my expectations. I base this on past history of buying expansion packs and comparing what was offered to the DA expansion pack. DA kicks some serious butt in this category with all the improvements in graphics, additions, and overall game play.
That being said... as with anything there is room for improvement. Everyone has an opinion based on their style of game play on what features they would like enhanced or added. I play on huge maps at the painful level so these comments are based on that type of game.
The 3 things I would like to see in future additions would be:
1.) Improved Land Based Combat. -
A.) Please at the very least upgrade those cheesy graphics in future updates. I feel like I am in a time warp back to the early 90's when I see the land based combat. Hell just put on a rockem sockem robots commercial (am I showing my age) in its place for entertainment value.
B.) The combat is overly simplistic and there is so much that could be added to enhance it. Separate out soldiers from the regular population base so that you can have more alignment specific choices. If you opt for a genocidal victory there should be some serious evil/influence alignment penalties along with degradation of the planet's quality or industry base. If you are good and keep the civilian population then you might have a more intact planet after conquest but be subjected to revolts, poor morale or other types of issue. Your influence might grow however as other races see you as a more benevolent dictator rather than some genocidal maniac. Again these are just ideas to show the possibilities. As it stands I go from attack to skip as fast as I can. After the first couple of combats does anyone actually sit there and watch the display? Come on be honest now!!
2.) Improved spying. This is overly simplistic and needs a little depth. Its going in the right direction just needs more enhancements. Give your spy's more to do. Perhaps even start giving them experience based on how long they are on the job without getting caught. Lots of things can be done just think that currently its a little barebones.
3.) Improved AI - There are quite a few area's in this category so I will limit myself to one that is a pet peeve.
Planet Development.
I think the AI is smoking crack when I see what they are building. I use this as an excuse for my good alignment to justify the genocide portion of the game. Better dead then a planet full of junkies!! The AI will have a technological capital on a PQ6 planet with only 1 lab 2 factories and a starbase and an embassy

In most cases it will ignore bonus tiles building random improvements on them. The AI will then leave a PQ20 planet relatively undeveloped after sinking recourses into developing the more useless planets. I am not sure how it formulates what to construct but I think that a balanced template should be implemented so that each AI planet gets a priority list of things to build initially then let it muddle things around the best it can later on.
On the smaller PQ planets I would build factory/market/entertainment center/basic farming. In this way even the smallest planets would be self sustaining and offer moderate defense with respect to having a higher population that could thwart a transport or two depending on your soldiering ability. The factory I usually upgrade to something else farther down the line (market perhaps for a higher positive cash flow) but I would not expect the AI to be that advanced. After these 4 items then let the AI kick in with whatever madness it does.
I would limit the population centers the AI builds so that the typical 30 billion max planets are thinned out or if it builds population centers have them do it on the correct tiles. Why build 3 or 4 population centers on everything but population bonus tiles then build a lab on the 300% bonus pop tile.
I am not sure how the AI survives without cash but if I built economy tiles like the AI does I would be bankrupt before the colony rush is over and no hope for stability later on. Maybe my approach to the economy part of the game needs work but I honestly cannot see how some of these AI races have a viable economy based on the build of their planets. I think there is a magic ATM supplementing their income somewhere in the mechanics of the game but I could be mistaken. Or perhaps this is why the econs seem to implode for the AI in the mid to later portion of the game. For a human (at least this human here) its the opposite. Struggling econ at the beginning of the game but thriving as time goes on.
Much as I would like to put a few others down I will leave that up to the community. It would be interesting to see a list of the more popular changes be put into a voting window. This might help drive the direction of progress for future updates/versions.