I can see Phase working, although that'd cause a little discontinuity between what Phasing does (i.e., outside space) the Phase Beam <sigh> and all that.
I still think High / Middle / Low Tech might be the best long-term solution. But that'd be a whole new Mount type, and I can only imagine the flame-wars that would ensue if that got changed now.
On the other hand... an intriguing idea... maybe mounts could mount the Tech levels below them, but not above them? So Low Tech can only mount Low and would correspond to Ballistic, High would be equivalent to Universal, Mid would be equivalent to Energy but could mount Low Tech?
Or it could work more-or-less how it works with mount sizes, where one unit down can be mounted, but not two? So High could mount Middle, but not Low, and if there was still a genuine need for Universal, keep that in?
Fairly long tangent about the balance issues that arise from the current system and stuff.
Spoiler
That'd sure put a different spin on how it works now, but the arguments over the game-balance issues that would arise would no-doubt give Alex a serious migraine.
It's like a lot of stuff that I've effectively modded out of Vacuum atm; I really don't see how, in a really balanced game design, we need all these barriers to player choice or how it's really enhancing the game's design or makes the game more Fun. I don't mean to come across as super-critical in that sense, it's not a crippling issue, I just feel like it's something that could be cut and it wouldn't suddenly ruin the game.
As an example, all of the classes in Torchlight can use practically all of the weapons, armor and shields. There are a few exceptions to this, but they're end-boss unique gear things. Otherwise, you can use anything with anybody.
Yet generally speaking, we don't actually end up doing that, because each class has its own nuances and some weapons don't work as well as others. Plus there is that other selection criteria, style. Maybe my Paladin is numerically better crushing stuff with giant hammers, but I really like how he looks using his sub-par-but-visually-cool sword-and-board skills.
So in essence, eliminating hard walls, in terms of barriers to choice, doesn't mean that we're not going to want to push our "characters" towards a sensible opti-max; if we're flying a Tri-Tachyon vessel with the Flux Dissipation to support lots of High Tech weapons that cost more Flux but don't have ammo problems, we're going to go that direction out of self-interest, not because the game forces us to.
I'm not saying that's easy to figure out. Part of the issue here is that the High / Mid / Low ships aren't necessarily pushed far enough away from each other that, for example, a Paragon is a shield-tank, but not an armor tank, opposite for the Onslaught, with the Conquest firmly between, etc., etc.
Part of that issue is the universal nature of the Flux system, which has always been an inherent penalty for Energy weapons in general; it's always been pretty problematic that Flux drain is effectively shield-killing damage over time, it's a much bigger problem than Heat management was in the Mechwarrior series.
I really wish we could split Flux up into weapon Flux and shield Flux and assign the relative number of points of Dissipation to weapons and shields, because that would fix a lot of the things that are causing issues with trying to arrive at good balance and vessel differentiation / tech styles / good, well-balanced pigeonholes atm.
Anyhow, sorry for the long tangent, it's just all inter-connected.
Mount sizes have always made sense from a functional standpoint but the arbitrariness and quasi-elemental nature of the mount types that the current system has created doesn't really help much, other than keeping ships pigeonholed, often to the point where there simply aren't any competitive ways to equip them. So they're unique, but I'll never fly 'em unless I'm really bored, and they might as well have static equipment lists, because the only time I'll see them is when I'm blowing them up.