Demigod's been really fun experience for my gaming group. Because of all the press about the bumpy launch, my friends and I got into it later than most. We play a few matches every week.
We don't play the ladders or PvP, so I don't have much to say about those topics. We play casual co-op -- humans vs. AI teams.
My top choices for something like an expansion or DLC would be:
1. More maps. (Or release the tools so that users can make our own maps.)
2. More Demigods. How about a giant skeleton king, a land-based octopus-type creature, a scorpion or spider lord, or even a whirling tornado of destruction! Rar!!
3. Assignable AI for computer-controlled Demigods. It'd be great to select a Demigod and then apply an AI profile to their gameplay.
4. Fix the pathfinding snags where Demigods get caught on geometry. (Yes, I'm looking at you, Crucible.)
Other changes that would be welcome for a more feature-packed update:
- Better visibility of smaller Demigods in crowded battles. We've had many a time where one of our Demigods goes down because we couldn't tell that we were fighting two enemies at once.
- More intuitive controls for minion management when playing a general would be most welcome. We're always struggling to have the correct unit(s) selected (or not selected, as the case may be).
- A real-time updated display of currently summoned minions would be very helpful.(Unless I missed it's presence on the UI somewhere?)
- It'd be helpful to have a choice in the Options menu to pause the game while still being able to use the store menu. That probably seems silly to PvPers, but to my co-op group, it'd be most welcome. The enemy AI always knows exactly what it wants to buy and can do it instantaneously. We would prefer to have a pause where we can huddle up and plan our upgrades and gear.
For Demigod 2, a few ideas...
- Instead of populating the map in advance with towers, have an option to allow players to place defenses on the map by choosing from a pool of structures to defend their Citadel. Sneak a bit of the Tower Defense mechanic back into the player hands if they want to use it.
- Multi-level maps. Ramps, underpasses, etc.
- Increased level cap with extended Demigod trees for those truly "epic" matches that go the distance.
- Give towers their own upgrade paths, and the ability to upgrade specific towers at the Citadel or from a new "war room" building. Consider building a few new Demigods who could be useful as "Citadel guardians" who watch the home front and coordinate upgrades to all defenses.
- Weather/environmental elements that can actually affect battles.
- Maps with some paths segments that are destructible, but can be rebuilt. Tie the destruction to a negative effect (e.g. gold loss) and the re-build to a positive effect (xp or gold bonus) so that the defensive maneuver of destroying a path has a consequence and the enemy will be well-rewarded if they rebuild. In the meantime, though, forces will have to take an alternate route.
- Better net connectivity, ability to join or re-join games in progress, etc. This has all been mentioned already in other comments.
- Shift-click to cue up waypoints for Demigod or minion movement.
- Add an optional tithe system where players can set a percentage of all their earned gold to go to the Citadel treasury (which again, could be a new building or part of the existing Citadel). Upgrades to the Citadel can still be initiated by any team member, but the upgrades are paid out of the treasury funds.
Hope you find some of this helpful or at least entertaining. 
