The scenarios idea is interesting, but as you said probably is better if I keep it as expansion pack. I have already lot of ideas right now about the game, but I'm deciding now what can be included in the main game and what is best kept as expansions.
An example is heroes: similarly to Master of Orion 2, or more recently, Civilization IV, you have some special heroes (that aren't only generals but also scientist, etc) that you can hire and place on certain star systems or planets to increase some values. For example a hero scientist could boost the science value of that planet/starsystem, and so on.
Regarding the A.I., I want to implement an interesting thing: a difficulty that automatically adjust based on player's skill. That is, if you play badly, the game becomes easier, while if you play well the game becomes tougher. This way I should be able to prevent too easy battles, which in the long run can be boring and reduce the gameplay value.
Since I'll be using for the map fog of war I'll be able to make some interesting A.I. choices like sneak attacks from the computer, and so on. The simulation/management part probably can be done in 1 month of work, what will be very hard is the battle itself, because I want to make a very good turn-based wargame