I usually shy away from posting feature suggestions because I'm very much aware of the development time, so here goes one I know is simple to implement.
Taking into account some other features (such as mixed race missions), I think it would be interesting to occasionally have partial intelligence reports on alien missions, somehow tied into the Geoscape. X1 has the quantum decryption, as did the original game. That's relatively late in the game, and provides full intel. My suggestions would be to also occasionally get partial information about the upcoming mission, consider it information from civilians, local military flights, and so on.
Examples of such intel could be race info ("aliens have deployed robotic units on this mission", "a reconnaissance flight has spotted Sebillians"), numbers info ("a local outpost reported at least 8 hostiles before we lost contact"), and possibly objective/aggression/map info, whatever else fits with the combat.
These should not be available always but should, when they are available, be helpful in preparing for the mission in the best way.