One of the problems I have with the X series and why I never really got into it is the way that they combine first person spaceship flying and economic builder/fleet manager. I like both genres individually: Elite Dangerous and OpenTTD (with the FIRS addon, which is quite X-ish in how the economy works) are both games I've played a lot of. But I find that when the two are combined, it ends up with
- the asset and fleet management strategic side being made irritating by the need to fly around to do stuff rather than having a proper top-down multi-window interface that can control it properly
- the flying around side being interrupted by the need to go micro-manage something, or get involved in the strategy side to get the resources needed for the flying around bit
- the timescales of the two not quite meeting up: it can take hours for the freighters to move around ... whereas the flying is genuinely real-time.
A separate RTS or Economy Manager game set in the Elite Dangerous universe with a lot of the same parameters - but sped up - that could be very good. I don't think trying to fit it all into the same interface would work well, though.
In practical terms, I think balancing this against the rest of the game would be extremely difficult. If the activities of your AI ships are generally profitable, then it's not hard to set up some infinite money loops by getting a bunch of them to mine in an uninhabited system, sell to some quiet fringe system where they won't be disturbed, repeat, use the profits to buy more miners. Maybe you'd also need to buy some escort ships so that the (abstracted) result of the battles this fleet had against pirates were all victories without you needing to personally escort the miners. Once you have infinite money - or at least earnings faster than you can really spend them - you can then throw that around with less profitable stuff like combat ships to influence the BGS and cut basically everyone else out of the game. There's enough complaints about BGS bots already without Frontier providing a "get your own very large fleet of official bots" mechanism to allow a small number of ultra-wealthy commanders to overwhelm any normal player activity with no real scope for reprisals. (In a single-player/small multiplayer RTS spin-off this mechanic would be actively desirable and not a problem at all!)
Limiting the number of support ships your fleet can have doesn't necessarily help with this - at most, it slows it down.
On the other hand if the activities are not profitable (and there only needs to be one profitable "recipe" for everyone with a fleet to set that off in the background), it's just
more grind for you personally to keep the fleet's balance topped up with void opals.
There's also big questions around "if your fleet does something, but no player is in the instance, what happens?". If two AI fleets meet when no player is around to see it, what happens? A dedicated combat pilot with a well-engineered ship could probably wipe out several fleets of NPCs while their owners were asleep (massacre missions are basically just that) - balancing the feature between "gain infinite power quickly" and "your entire fleet was wiped out by CMDR Bob while you slept" would be really tricky.