We already have all the components for espionage in-game. Hell, when you wardec someone, you check the other player's relative fleet strength, and alloy storage BEFORE you go to war with them. Thats FREE espionage right there. You check the opponent's alloy count every once in a while to make sure their alloy storage doesn't drop from 5k to 1k, and if it does, its your cue to build a fleet.
You can currently see:
Alloy amount/income (trade another player, to see their alloy amt/income)
Relative Fleet Strength
Relative Research
Relative Economy
You can currently see:
Alloy amount/income (trade another player, to see their alloy amt/income)
Relative Fleet Strength
Relative Research
Relative Economy