Should be able to buy a megaship, not just a fleet carrier, for specific roles.
Maybe when Colonisation opens up those bulk haulers or research vessels might become available.
‘Fleet Management’ could be a thing too as a result to aid efforts across the galaxy.
Hows megaships would be better that FC's in any given role? With that limited mobility of single jump per week.
Higher jump range? Good luck with one jump per week, even if it would had jaques station-alike jump capacity (after all it did jump from bubble to colonia in one go, back in the day), but image tritrium required for that, not to mention, of breaking jump range balance in game, as "unlimited" jump range would mean such megaships would be everywhere (given enough time), and surely would made concept of travel meaningless, to any distant part of milky way, effectively making scale of game virtually smaller.
Only good thing I can think of that, it would be ability to reach otherwise unreachable systems with means to go back, and there is plently of those, like specific star cluster (the famous PSR) that is so far from the galaxy map, that cannot be zoomed in, and very few cmdrs was be there only because during betas where they been send to it by Fdves.
Better cargo capacity? Now, even with 5 times much as cargo capacity, that 1 jump per week still makes it quite slower than regular FC bulk cargo runs, as wing of trade cmdrs can load and unload entire carrier worth of cargo in few hours.
Given those limitations, and since FC are already there, megaships would be nothing else that peety place holders in order to brag about one's wealth.