I modded in a slight change to the sprawl penalties just for experimentation and confirm my suspicions.
I added -1% Subject Relative Power Opinion per 2 points of sprawl.
Just having the one green number getting better while the red ones got worse made the increase in sprawl feel like a neutral thing instead of a bad thing.
(quoting this from the main feedback thread to discuss)
I think this might actually be a really fantastic idea if for no other reason to reduce the "Oh no, sprawl is evil and must be defeated" reaction people are having. Right now, sprawl only has negative effects, so people are obsessive about trying to keep it as low as possible even in situations where it doesn't make sense.
As an example, going from 500 to 1,000 sprawl only increases your tech costs by 33% (~150% -> ~200%), and for unity, that same increase would be a 50% increase (~200% -> ~300%). But you've
doubled your empire size, and thus, if you aren't neglecting your research and unity production, you've doubled your tech and unity output. In terms of time it takes to gain a given tech/tradition, the empire that's twice as big with twice as much tech/unity output will get their tech in 67% of the time and their tradition in 75% of the time compared to the smaller size (incidentally, without sprawl penalties, both of these would be 50% of the time, and reducing that difference is why sprawl exists).
What this means is that it isn't about eliminating sprawl entirely or even minimizing it. If you want to aim for tech and unity, what you want to do is increase your sprawl in intelligent, efficient ways where you can get more output out of your empire for a given bit of sprawl. If in doubling your empire size, you can get 2.5 times or 3 times as much tech and unity, you'll be better off, and things that reduce sprawl or provide output without increasing sprawl help that.
To come back to the quoted suggestion, I think it helps by providing a green UI number that people can fixate on to shift player perception. "Your empire has more pops, more planets, and thus more everything, so you're better than smaller empires, penalties or no" doesn't seem to be enough of an advantage for people, so having a few things where you have a clear bonus from being larger would be really cool.
I think subject power relations are one cool way, but I think there's more places that are less situational. Sprawl could be added as another source of diplomatic weight. It could factor into victory points or your economic strength. It would likely need to be things that aren't too overtly powerful because again, more sprawl already means more power. However, something in the UI to mitigate the feeling of "Oh no, sprawl is bad" would probably be really helpful. It might be a bit awkward to have various sprawl reductions have negative effects, too, but I don't think that's a huge deal. For example, having an empire with an unruly population have more political clout than a similar sized empire with a docile population makes perfect sense. I think the traditions are the main places that might need a change, or at least have the sprawl-reducing traditions move into one or two trees that are more thematic and empires that don't particularly care about sprawl can just skip those in favor of other trees. Any positive effects of sprawl could also be based on the pre-reduction sprawl. Maybe they could have two numbers, sprawl and size where sprawl is modified by the various traits while providing the penalties, and size can be the same thing but ignoring any modifiers and provide the positive benefits. That way, stuff that reduces sprawl is always still good, but your empire's size is better understood as a positive thing, so overall, you still want to be bigger (you just want to be more efficient with that size where possible if you want to emphasize tech/unity).
Oooooor they could continue to tweak the UI and make more red numbers yellow or white and hope that over time, the player base gets it. I just hope they don't knee-jerk react to all this feedback that's more about misunderstanding the game than anything else. That's how we got bureaucrats in the first place, and we all know how that turned out: not well.