If I had to guess, a bigger Heron, like how the Champion is a bigger Hammerhead or the midline capitals are bigger Falcon/Eagle.
Legion is a battlecarrier.
Astral is a bomber carrier.
So, a non-bomber focused capital-sized carrier (that favors mostly interceptors and heavy fighters) would be nice.
Heron is a cruiser and does not count.
Midline could use a proper capital-sized carrier. But... high-tech could use destroyer and/or cruiser-sized carriers, but since the primary high-tech faction will focus mostly on warships and phase ships, there does not seem much point to giving them ships they will not use. Similarly, what faction would use a midline capital carrier? League seems to be set already. Maybe the Diktat since they only have one capital, a Pegasus knockoff. But Diktat feels like an aggressive faction, not one to hide behind fighters.
Aside, the one faction that could use (another or replacement) capital (and a second cruiser) is Derelicts. Guardian looks like a limited special boss that get shoehorned into Derelicts when contacts needed a capital attached to Derelicts to make those bounties bigger. Would be nice if there was a Derelict capital more on par with Atlas/Prometheus II instead of a Ziggurat-tier monster.