What *doesn't* go in city blocks?

I'm done with Aquilo, at about a (mostly) steady 1.5k eSPM, and I'm ready to start really scaling up properly. My goal is gonna be 1.8k real SPM, half a fully stacked green belt for each pack. I haven't done any megabasing before, after some thought I want to do a train based city block base on Nauvis, with direct insertion from miners/foundries to wagons and wagons to production buildings as much as possible. Minimum use of belts, minimum use of bots except for unloading the cargo pad and those little annoying things like calcite and train fuel.

I think my next step is to figure out the high-level details like grid size, rough macro layout, bot segregation, how to do circuit shenanigans for train scheduling, etc. Total demand & how much production I can fit in a single block, how to fit beacons in when doing direct insertion to and from wagons, how to manage trains entering and leaving blocks without affecting through traffic...

Thinking about it, I feel like some things don't make sense to go in a single block - labs and the cargo pad, nuclear power (fusion probably could but I don't think Aquilo can supply enough fusion cells yet, and I'm kinda sick of that place), rocket silos? I guess blocks that produce a good that gets exported could have their own silo, I can't really see a downside to that.

So, what doesn't go in city blocks?