Actually i'd suggest it to be implemented (if it would be implemented) as a training option in the game.
Add a "holodeck" or something research option somewhat into the middle game (i suggest a followup after alien entertaintment research?). Which would allow you to build a base module.
This would allow X-Com to play training missions as an option from the base menu.
You would be able to choose any weapons and items X-Com has access to or researched, any mission kind you already played (alien base, crashed ship recovery, live ship recovery, terror) on any terrain you already played on (desert, farm,...). Ship missions would require a specific number of said ship missions (based on size) or a research topic on the said ship as well. Enemy race also must be researched (and to allow enemy use of psionics, psionics must be researched). Enemy will also use only weapon sets researched by X-Com.
Those missions could even be giving experience to the soldiers who play, probably at lower rates (or lower rates with each subsequent mission they took part in)
To counter the benefit of giving experience, they could cost money to play. Should be balanced so that its not possible to train your people into supermen, just train some rookies into squaddies with adequate stats so they can stand for their own when base defense comes.
Then, after the game end, possibly unlock training option in the game menu, and subsequent played games should add options to this mode (so if you gain acces to another training mode in another game, that would be added to the main menu option).
Thats of course just a thought. But what a great idea !