IMO there is nothing gameplay wise that a module based tank designer can add that the old variant system with minor tweaks could not have done equally well if not better. You can design any conceivable stat tradeoff into the variant system equally by adding a few more +- sliders to cover the aspects the designer tries to cover and spending some time thinking and testing them so they are well balanced.
A variant system could just as easily be connected to research as well so that you need certain levels of AT or Artillery research before unlocking certain levels of variant improvements to Tank/TD or SPG guns for example. Ofcourse this is not something modders can do so I agree that this is something that limits us, but I doubt it would be as hard for Paradox to do as making a whole new system from scratch and then adding a connection to research into it instead.
What the levels-variant system cannot do for AFVs (and even more for 'planes, actually, but it's bad enough for tanks etc) is allow for designs that fit different use cases than the "standard". I ended up adding a couple of extra tank classifications for Waltzing Matilda, but that was really only a bugfix/workaround. With aircraft it gets worse with theatre specific requirements and adaptations to fit enemy capabilities and terrain difficulties, and these were explicitly strategic concerns which resulted in decisions about particular manufacturers and factories being dedicated to particular theatre supply in some cases. The variant system leads to some models being way more useful than they should be, while others lack the flexibility that they enjoyed in practice, and in some use cases being absent or irrelevant in the game. Germany made more assault guns than tanks in the latter stages of the war, and yet the use-case for them is simply absent from the game and unfixable by the variant system. Sure, you can mod them in, but then tank destroyers are a totally different technology, which doesn't really reflect the reality at all.
Basically, you can kludge quite a lot in, but there comes a point, when you are hammering screws home with a hammer, that you just think "You know what, it'd just be much better if I went and got a screwdriver!"
Should be easy enough to do a reskin of the ship designer for aircraft, they are already working on that for tanks rn
The reskin is easy; the hard part is understanding enough about 'plane design tradeoffs and possibilities to make a model that actually makes sense. In addition to which, I think you'd be wasting time if the air combat system wasn't adjusted first to account in some way for altitude, navigation, blocking defences and differentiation of targets (surface ship/submarine, armoured/soft/fortification - for example).
I would strongly prefer not using the MtG navy design system, I already find it too unnecessarily complex for ships and adding it to tanks (and probably planes later at this rate) just adds a ton of complexity with little fun added unless you love this thing, which I don't.
I get why it seems great on paper but in reality It means having to waste time on forums trying to figure out what designs work since the auto designs are horrible and the game doesn't clearly tell you what is smart. It will just be about following whatever is meta.
If there is one single "meta" the design has failed. The difficult part should not be making a unit with the attributes that fit the design brief as best the available technology can make it - the difficult part is
deciding what you want. And that should be different depending on what you are trying to do and where you are trying to do it. That's where the "strategy" bit comes in.