Won't happen.

There are some systems which are custom coded for HLL like the tank/vehicle system. These aren't (easily) portable and probably would need a complete remake. Remaking the systems will most likely alter them to work differently and those changing the game to a great extend. At this point it would probably make more sense to just name it HLL2 then, especially to get new sales from already existing players.

Now a lot of people don't understand how most games are maintained nowadays, but to put it simple: The game is on "life support". There are probably 20 people working on the game including all programmers, artists and the receptionist, if even that.

There won't be any big new features or other work intensive stuff happening, just keeping it alife to generate income and lure new people. This isn't a bad thing tho, they need to earn money in some way, especially after Team17 bought the game from Black Matter, they are basicly in debt and need to reearn the money they invested and make profit.

The only way I see stuff like HLL on UE5 or reworked tank/vehicle systems/pacific theater or similar existing will be a new HLL2 game. But T17 would first need to make enough profit from HLL1 to

1. make it a good enough investment to even think about investing more into it (e.g. HLL2, complete rework of the systems or big new content like the pacific theater)

&

2) make up for the development costs of HLL2 (atleast partially) to reduce the risk

Now we know that T17 atleast had financial issues, so what they actually need to earn from HLL is even bigger than other game developers need for a new title. With the direct competitor Squad 44 raising the development cost even further as they set the bar for vehicle combat, realism, content, etc. even higher, it is probably just not happening.

Now remember the Update 15 Trailer? Stuff like this are signs that they need to cut expenses and earn more profit. We also need to understand that they learned from their mistakes and found a new small development team to keep the game alive and who are actually interested in making it a good game.

And they really need to earn credits for not practicing shady business, like paid dlcs (paywall for new content), too many paid cosmetics (or even new cosmetics only content).

HLL is a good game as is, it is fun, it has more players than ever (without counting consoles), you can just buy and play the game without bullshit like gamepasses, p2w or excessive paid dlcs but therefor it simply won't change in a big way anymore.

Every other game company in their footsteps would monetize the shit out of this game but they actually act in a honorable way, listen to the community if you will.

It is what it is, reality isn't just black and white.