Another thought I've had, after poking around my late-game colony some more - with especial regard to the mega-domes, the current biggest size, there are inevitably some buildings you have to double up on because most of them have a very limited capacity to serve colonists (5 or 10 at a time - the spacebar is the worst here, it takes up a whole wedge and only servers 5). For higher pop domes, this becomes an issue that drags down the comfort level overall because even though you have the right buildings to service their needs, the colonists just can't use them because they're always full. Thus, it would seem logical to introduce building "levels", perhaps locked behind dome population or perhaps simply a technology, whereby you can upgrade an existing building to essentially have a higher capacity for servicing colonists (and which would also of course employ more colonists too).
This would partially solve the massive unemployment problem I alluded to in my original post, where I mentioned that my colonists just keep on having babies (3k population is within sight now) and I've been building extremely housing-heavy domes as fast as I can to try and alleviate it - which end up needing multiples of certain buildings, as I've said - but even so, what employment buildings I need to add to the dome end up being woefully inadequate in soaking up workers, to the point where I spam out-dome buildings like fusion reactors and polymer factories, not because I need them, but because there's nothing else I can do. I've even had a bunch of techs that can reduce the workers needed in certain buildings that I've never ever made use of as it's literally the opposite problem I have.
So, building levelling options that increase the number of colonists served at a time, the number of colonists employed, and cost that bit more to maintain - in retrospect it seems such a no-brainer that I'm not sure why it's not something I thought of to begin with.
(PS - I like a bunch of the other suggestions so far in this thread, by the way!)