This is what I'm talking about : (screenshot)

That sounds more reasonable, but still has many problems.
You would need to have made these changes in beta, as at launch the game was already not setup for the Hex system with all problems that entailed. That said the devs have shown their ability to make drastic large-scale changes to the game, however flawed those changes were. So lay it out: what rules would you use to ensure your open ended system works?

You need to overhaul the way spawning works to make logistics worth anything. But still make it easy for players to do the things they want rather than spend most of their time traveling through empty landscapes.
You need to look at hoe players can travel through frontlines, as in many area's its nigh impossible to detect and destroy enemy incursions behind your lines. Either the defenders need more tools and methods to detect and intercept them or the attackers need less options to get around.
You need to have better methods of communication and incentives for players to find and spawn at new Sunderers. Especially if you are going to make vehicles expensive it would be a bad experience for players to get a Sunderer to a spot and like now players wont leave their fight to join the new one. With more expensive vehicles the defense of Sunderers would become more important but few people enjoy the long wait at a Sunderer to defend it against something that may never arrive. You need to make it fun to wait at a Sunderer to defend it.

Your idea of more expensive vehicles is also nightmarish without more changes. Again in the old days we already had more expensive vehicles (more time to refill your vehicle nanites). This caused horrendous snowballing. One side would win the vehicle engagement and become the attackers. They would have a bunch of vehicles and a good spot to gather their vehicles. The defenders would have just lost a bunch of vehicle resources and have no vehicles at their disposal, making it easy for the attackers to maintain vehicle superiority. So you would need to make sure the defenders can actually defend against these vehicles somehow and push them back enough to build their own effective vehicle force or deal with the vehicles some other way.

So, what would you change and add to ensure the game actually improves when they add a fully open continent?

Well... I'll give it a go, for fun.

Make it harder to fight the further you are from your warpgate:
- air and MBTs spawn from the warpgate (buff MBTs to compensate ?)
- respawn time longer the further you are from your warpgate

My personal favorite feature that nobody wants but would help immensely to balance the game: cap the number of vehicles each faction can spawn at the same time. If your faction overpops, you'll have a harder time pulling a vehicle than if you're underpoped. I mean... how comes faction factories can build more tanks just because they have more soldiers ??? This has always bugged me. To the point that when I started playing back in 2013, I ASSUMED the resource pool was common. ^^

I think these modifications push the game towards a more balanced (static?) battlefield where the 'natural' position for the frontline will be halfway between warpgates in kind of a status quo. Enters the fully open continent that will bring some diversity and change of pace.

Anyway since I like my gameplay slow and methodical, rather than fast and chaotic like the majority of players (I'm old ^^) I'm sure it will never happen...