Things like "Intel" like in HOI4, where you can see where enemy units are and such if your decryption stat is high enough, or at least significantly higher than the enemy's encryption (I admit it, though, I'm not sure how intel in HOI4 works, to be honest

)? Sure. Covert stuff, perhaps implemented as event chains? Quite possibly, if implemented well. Secret CK2-style cloak and dagger scheming? Absolutely.
"Espionage" like sending spies on missions, sabotage, and so on? I'm highly sceptical, and I'm glad I'm the only one.
In fact, I think CK2 nailed "espionage". Stuff like assassinations and other cloak-and-dagger stuff isn't done by generic "spies", but by potentially anyone -- your family members, your friends, and your loyal allies, Song of Ice and Fire-style. Having to deal with the illoyalty of your admirals, generals, guvernours, and other staff members, like we're going to have to with characters' political "issues" and the faction system in general in 1.5, sounds
so much better than generic and, to be honest, pretty clichéd "espionage".
Edit: also, as others said, counters. I don't mind major stuff, even "Star Wars"-level stuff where a battle station loses its shields temporarily due to sabotage, but there has to be a way to prepare for these things, and detect them when they happen so you can counter them. The same way you counter enemy ships, you should be able to stop enemy spies and commandos. Also, I don't want this implemented as an "espionage" system, but rather as rare events, probably implemented as "story arc" event chains.