It would need counter play. Stuff without counter play is fundamentally frustrating and unfun. No idea if you remember early Synth uprising. Where they would constantly scuttle star bases and the likes. It was infuriating and unfun to the extreme. This is that, but being done by players and worse. You'd also have to give the other side the same tools, i.e abduct pops, cause low stability, and so on.My hot take is espionage needs to cost specialist jobs to be good so the damage you're theoretically doing is coming at a cost. Sure, you can cause damage without a war, but your military suffers through either technology or alloy loss so if someone figures out you did it your ability to defend against direct conflict is reduced.
Btw as for your other idea. How good or bad slavery needs to be depends on the risks. If it's just "different" from non slaver play style but balanced otherwise, it would need to have about the same output and beneficial civics. If it has these massive risks associate with it, and would cost extra jobs and other headaches. It would need to be genuinely more productive/stronger in output and not just base resources which are easy to get after a short time.
The whole "working two jobs" for example sounds good in theory. But you absolutely want maximum efficiency for specialist jobs. And jobs in general long term. So getting a 25% fine on them would kinda hurt you mid and long term. *Shrug*
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