Where it can go from here....

The best character is presumed dead, and if he somehow isn't, I don't know what more they could do with him? His character arc had come to a suitable conclusion with him a changed man with a positive outlook on life, before he was led away to his fate. If he returns then what? He either forgives Milky for it, which he's kind of already done, or he takes revenge on Milky which would simply be stupid after his redemption.

Apart from Milky the other characters have reached a positive conclusion. Any focus on Harvey or anyone else going through a drug problem would be overkill.

Only reason Shane Meadows could bring it back is to show Shaun going out with a supermodel this time.

Op, your wants and needs from needing closure, simply show that the entire point of the film and the 3 series, completely went over your head.

Noooooooo I can't have anything more bad happen to lol and woody! I don't think I would be able to handle it. They've had more than their fair share of darkness I think they should be left to be as happy as they can be.
Shaun getting the new GF pregnant is a good idea, you can imagine the outrage it would cause when Cynthia found out haha, you could go down that route with Shaun freaking out over the whole thing, but then he thinks about how hard it was for him when he lost his dad and so he wants to be there for his baby. But then, Shauns meant to be an alter ego of meadows isn't he, hence him going on to do the photography course and stuff, and so seeing as none of that really happened in meadows life I don't really see it happening...
I would love to see more of Harvey. And maybe trev as well? And I think it's about time gadget gets a nice girlfriend whether it's Kelly or someone else :))

Forward 20 years, Woody and Lol have grandkids names Lolz and Rofl......