I know custom scenarios are possible with the current XML structure, though I'm not sure if pre-founded colonies/pre-placed buildings are. Those sorts of scenarios could be an interesting way to add some more bulk to the single-player experience, but I'm not sure how much additional work they'd take to implement. Either way, I'm not sure a more involved challenge like that is really appropriate for dailies from an effort-exposure standpoint. Having a really good scenario limited to a daily would be a shame, after all.
That said, I agree that dailies can get a bit dull over time because even though maps and settings change, the basic outline of the strategy needed doesn't really change, in part because the AIs don't adapt as well as good players. Throwing in some of the modifiers that appear in the campaign (free extra buildings, no adjacency, etc) could add some variety to dailies without requiring too much new stuff to be implemented (probably). DeathTacticus and others have brought this up as well, but a longer-term leaderboard for dailies could also help keep those engaging. I'm not sure how it would value relative times or ranks (or missing some dailies), but yeah.