By filler, I mean stuff that you do that makes it feel like your proactive, but done in a way to ensure that the overall pacing isn't changed too much. Like I said, the big issue is that you're stuck twiddling your thumbs waiting for stuff to build up in order to do things. If one isn't into espionage, this is fine, but the moment you want to be more proactive, it's pretty awful. This is also factoring in that paradox is trying to avoid having a setup where it ends up being awful to play against, hence the whole no assassination thing. That said, I do think if they made assets cheaper to get, it would give plenty of room to say have operations to weed out hostile assets. So this is probably a factor in why smear feels so underwhelming because they don't want it to be super easy to bust up federations or alliances.
Asset operation should be cheaper, I've found that even in end game, if you go for a certain build, it's possible to luck people out if they get all the techs, but don't enable the edict. I don't think it should ever be at a point where someone is stuck unable to do anything, while they wait for RNG to give them an asset or having to invest valuable resources to ensure they never get hard countered like that. The strength with the edict and psionics shouldn't be that you always have a foot in the door. The edicts big draw should be you have another envoy and have to do less for base intel. Psionic has all the good stuff. I don't think either would be hurt if one could always have access to getting more assets with max codebreaking techs; especially, if there were operations that removed random assets.
I do wish we could use the system much more against crisis stuff. Not sure how best to achieve that because well the crises are trying to kill you. The failure to include them really undermines the espionage system.
Also as an aside, I wish the crises players had more to gain from this system. The poor interaction really undermines the playability of the become the crisis perk. Unless someone wants to take forever to hit crisis level, it's not feasible to really earn most of your menace points through this system, it take far too long and it's a shame. I would have loved it if we had to converging paths that led to the same goal for the perk. The obvious one is being hyper aggressive and warring your way to victory, and the one they kind of keep from happening is the more more cloak and dagger approach where you use espionage and diplomacy to essentially build yourself up. Obviously, both converge on galactic war because espionage and diplomacy will only buy you so much time in regards to stopping the galaxy from going after for trying to destroy it. I will say for solo games, there really should be an option to make use of your currently useless envoys to maybe coax some factions in the empires you're fighting to rebel and join your cause. I wouldn't put it in it MP because that could get really annoying to play against, plus if every player is reasonable, if the game feels like it's dragging, players can come to agreement on how to speed things up to the conclusion.
Really, the big thing the system currently suffers from is too much thumb twiddling and not a ton of feeling proactive and useful. Assets being made cheaper, operations to burn those assets to build up the rate of infiltration for better operations and operations to remove hostile assets could be the right setup to make the system feel proactive and useful, but also avoid an issue where people hate playing against it.
The secondary issue is that it doesn't jive with the pre-level 5 crisis path or have any use for level 5 crisis and IMO that shouldn't be the case. At least in solo play, MP is it's own can of worms.