Midline Bomber
While there are a variety of midline heavy fighters with decent anti-ship capability, it simply isn't the same as the sheer alpha-strike potential of bomber wings. Even low-tech fleets have a bomber, midline fleets should too.
It's my understanding that the current consensus is that the Piranha is a midline bomber, not a low-tech bomber. This is generally supported by the coloration of the non-red portion of the Piranha's hull, which is more similar to the coloration of the Thunder than to the coloration of the Talon and the Mining Pod (more easily seen by examining the sprite in the game data files than by looking at the bomber in game, however), and by the color of its engine exhaust, which doesn't seem quite as red as that of the Talon and the Mining Pod, as well as (more circumstantially) by the description of the Conquest battlecruiser. Given that it's also similar to the Broadsword in coloration, though, I'd be perfectly willing to place Piranhas as either very late low-tech or early midline, particularly as I feel that Conquests are themselves fairly early midline warships.
If I'm not mistaken, the only fighter craft of the low-tech grouping are the Mining Pods, Broadswords, and the Talon Interceptors. The Piranhas, Warthogs, and Thunders all belong to the midline, and the Wasps, Xyphos, Daggers, Tridents, and Longbows all belong to the high-tech. I do agree that it'd be nice to see a bit more evidence of experimentation in the midline fighter craft, as this is when they became popular and when they should perhaps have many odd configurations which were tried and judged not worth pursuing, leading to the more standardized set of fighter craft in the high-tech grouping.
High-Tech Destroyer/Cruiser-Sized Carrier
Given the lore behind the Astral and the variety of powerful, shielded, high-tech strike craft, the lack of a high-tech carrier below capital level is really apparent. A number of missions involving high-tech fleets have to settle for the Gemini, which is midline and not even a dedicated carrier. A high-tech destroyer with 2 decks, or a cruiser with 2/3 decks, is almost desperately needed.
I don't know that this is necessarily problematic. If the fictitious space navies of the Starsector universe parallel real-world navies in their funding allocation, then during peacetime they will most frequently go in for building capital ships rather than the smaller vessels that can more readily be built rapidly should a war come up. Plus, the Heron is already at the edge of being a high-tech carrier with its heavily energy-focused armament and its more rounded and irregular hull shape than the other midline cruisers; paint the hull blue and I'd have been willing to call it a high-tech carrier. There are also some indications that the high-tech line of ships was developed in peacetime, especially with the upcoming rules change that removes hull logistics cost from the supply consumption equation. Lower crew requirements and lower ship upkeep, combined with the use of weapons which might be less effective than the ballistics they replace but which only really require energy from the ship's power plant, are the kinds of trade-offs you might expect to see a navy make when the budget is its greatest enemy, rather than a theoretical or actual hostile nation capable of seriously threatening it. I do agree that the lack of a late midline or early high tech light carrier is a bit problematic, lore-wise, as the Gemini feels like an early midline light carrier which should have been supplanted by a later design, rather unlike the way the Heron feels like a late midline fleet carrier sufficiently modern to work with the high tech lineup. I also agree that it'd be nice to see a little more diversity in the carrier lineup, because while the Heron is a 'fast carrier,' the only things we have to compare it against are the two light carriers, which are equally fast on the main map, and the Astral, which is significantly heavier. It could be nice to get a late midline or early high-tech 3- or maybe 4-deck cruiser level fleet carrier with burn-3, but I don't really feel it's necessary, and it might be problematic to squeeze a 3-deck carrier in that isn't either overshadowed by the Astral and Heron or completely outclasses the Heron. 23.5 logistics for 6 flight decks versus 10.5 logistics for 2; pick something in between there for a 3- or 4-deck carrier, and you have to start wondering if you would have been better off going for the capital carriers or just sticking with the slightly lower number of flight decks granted by the Heron but having space for more fighter wings in your fleet.
Low-Tech Cruiser-Sized Carrier
I get that low-tech fleets aren't supposed to place much emphasis on strike craft, but the only low-tech carrier is the Condor which is a freighter-conversion. Gameplay-wise it does its job just fine, but thematically it seems weird there isn't a dedicated carrier design for low-tech fleets: they still field strike craft, after all.
Personally, I feel that conversions and hybrids are a fairly good fit for low-tech carriers; after all, that's how many early carriers entered service in the real world, as conversions of existing (though often incomplete) ships, or hybrid cruiser-carriers and battleship-carriers (which were often, in the real world, eventually fully converted to carriers, scrapped, or retired due to being inadequate in both roles). It'd be nice to see a bit more experimentation done with it, but on the other hand, low-tech already has both the Condor conversion and the Venture carrier-medium cruiser hybrid (the Venture is in my opinion late low-tech, as despite its coloration, its shape has more in common with the Dominator than with the Eagle, and its basic statistics are likewise more in line with low-tech than with midline cruisers, with relatively poor flux stats, an inefficient shield, a heavy reliance on its armor and innate hull strength, and an armament that is heavily weighted towards ballistics and missiles, though which has started to introduce energy mounts). The Gemini already fits the role of a dedicated early midline light carrier intended to replace the Condor conversions in military service nicely, despite its description indicating that it's a freighter, although it'd be nice to see something a little more durable enter service in the late midline or early high-tech lineups to help emphasize the increasing importance of the fighter and make the high-end carriers a bit more diverse.
Phase Destroyer
A destroyer hull seems to provide the perfect mix of firepower and speed phase ships require, yet while there are phase frigates and a phase cruiser, there is no phase destroyer. Not a huge thing but definitely noticeable.
Mid-High-Tech Destroyer
By "mid-high-tech" I mean in the same group as the Apogee and the Odyssey: high-tech vessels that still have some of that midline feel. I suppose the Sunder sort-of counts, but its design is more obviously midline than "mid-high-tech".
I tend to feel that these two could perhaps be answered by the same ship, though the phase ships do tend to feel like intermediary designs between the early high-tech vessels like the Odyssey and Apogee, and the late high-tech vessels like the Astral and Paragon. I tend to feel that the Sunder seems late enough in the midline grouping to have delayed a replacement design long enough for the replacement to come into service contemporaneously with the phase ships, despite the rather obvious shortcomings of the Sunder's armament (namely, that all the big guns are fixed mounts rather than being turreted). On the other hand, it also seems like phase ships were something that came and went rather rapidly, so perhaps the justification for the lack of a phase destroyer is that one was under development, but the advent of phase cloak-penetrating sensors lead to it being cancelled in favor of the Medusa.
Another possibility would be to create an early high-tech light carrier, thus filling some of the hole left between the Astral and smaller carriers, particularly the light carriers, as the Gemini feels as though it's early midline, and thus likely well on its way to being considered obsolete, at the time of the midline/high-tech transition, whereas the Heron feels more like it's just entering service at that point and can thus be expected to stick around for a while.