The way I see it, Ecumenopolis should have a district type called Commercial Arcology, similar to the Commercial Segments on Ringworlds.
It makes no sense for us to be unable to build up city worlds as trade hubs.
For gestalts, Commercial Arcology would be replaced with Reactor Arcology that provides Tech-Drone jobs.
Ecumenopolises are already extremely good for Rogue Servitors, with a special Sanctuary Arcology district. Realistically you would almost never use a Reactor Arcology as a Rogue Servitor, because the whole point of the setup is to concentrate Bio-Trophies in order to maximize *complex* drone output. Other Gestalts can't build Ecumenopolises; I suppose they could conquer one, but in that case, given Hive Worlds and so on, it's very unlikely they'd be short of tech-drone jobs elsewhere in the empire, so it wouldn't make much practical difference.
For non-Gestalts though I agree, Commercial Arcology would be pretty cool and thematic. Some kind of Administrative Arcology would also be cool for the "imperial core" theme, but this can probably wait until the unity/admin cap overhaul, at which point it could replace the Leisure Arcology (especially if you can spam Merchants, the circumstances where Entertainers are good are too niche to justify their own district).
