For towns: Always City Grocer (10% HP for all units, are you kidding?). Always Almshouse (needed on large maps with lots of cities). Always Guild Tribunal (don't really need the gold by the time my towns are lv5).

As for the merchant -> market thing, merchants can be built in all cities, market can't (town only). I think that's why they didn't make it upgraded straight up.

The potential traits are actually very powerful when stacked, don't under estimate them. It's the only way to get to very high levels.

Yes but unless you have a trait that specifically scales with level, being able to gain levels faster doesn't necessarily give much of an advantage.  There are some really good traits that you don't want to pass up if your character is focused on something in particular.

So the growth can be useful in very rare circumstances. Say for example you have several towns (each buffing the food in all your cities). You decide late game to raze an enemy city. This level 1 town is growing slowly. So you choose to add slums instead to get that extra growth in the city to get to the later stuff. The cap is not a worry at this point in the game and it allows you to rebuild a high level city quicker.

I'm not saying this is a viable strategy or the best one. It is currently my only strategy that I can think of where a slum is useful.

I use slums in Fallen cities when I plan to have access to Death V at some point. In essence it turns +growth into +mana (via sacrifice), fun and thematic! You treat people badly, encourage them to breed in vast slums only to sacrifice them later. People starting to starve? Too many people. If you care about your people you invest in good food and whatnot, but that's Kingdom philosophy only good if you're weak.

Also, the unrest increase + unrest from black market + unrest penalties after capturing the city make the city a miserable place to live that is extremely unproductive unless you're rushing production with whips and slaves where the population is regularly sacrificed... it's just a shame it costs gold to rush and we can't rush from population instead. (hint hint). (I do think Sacrifice needs an unrest penalty, there should be one)

Also, I have used slums when building newer cities that have access to lots of bakeries or a food resource to rush towards higher levels, so some people use them quite often. (Me)

I think Dr. Frankfurter pointed out the value of the slums and provided the exact game flavor for it. Although it doesn't fit in my gameplay style it is a building with a purpose... sacrifice your poor citizens for more of your power.

Yep.  That's the problem: there really aren't any choices.  There's one choice that's good, and the rest don't come anywhere near being as good--much less distinctive and contributing to a unique strategy in their own right.

Conclaves are the only ones that change for me. I play Epic, and I gingerly poked the XML to restore the faster build speeds that the patch changed to be more...epic. Now that it's pretty normal for a city to be able to build everything, the idle city benefits look much more attractive. I often take the 50% idle bonus to research because it actually has worth and valid use over the flat 20% research bonus now. On non-modded settings? What chump would actually take the idle bonus?

The big flaws are

- While idle bonuses are completely unfun, Derek is 100% wrong on this.   A few non-levelup buildings have this problem as well

- There is no thought into what building is best most of the time.

I think "when idle" bonuses would make more sense the higher they are in the upgrade line.