If Hop was still alive he would be EXTREMELY important cuz one:
He knows how to fight
He actually knows what he’s doing
And he’s smart and always finds a way
So I’m very upset he died, he was able to escape from the Russians and was able to find out the code with Dustin and Suzie’s help.
Some more reasons are:
He has Gunmanship: he’s good with firearms and knows how to use a gun (obviously)
He has survival instincts: his survival skills help him think about what to do when he’s stuck in a situation like for one being stuck in a tunnel in The Upside Down
He has Craftsmanship: in Hopper’s military life he was familiar with crafting
He knows Morse Code: Hop has some knowledge with radio operations using Morse code as he taught Eleven to communicate though Morse code by radio
If Billy was alive, now I’m not trying to be rude, but he would probably still be having an affair with Mrs. Wheeler. But besides that if he actually knew what was going on and had the time to actually ask how his sister(Max) is doing, then I would think he would help. Mostly because his skills are:
Enhanced Strength: after Billy was “activated” by the mind flayer he was able to bust out the sauna room.
Enhanced Durability: after being a little puppet for the mind flayer he was able to take hits and blows from anyone or anything, even El who blasted him out the locker room and through the wall. He was still alive(; -_-)
He also has a hive mind: when he was flayed he was part of the hive of the mind of, The Mind Flayer And was able to communicate to it(mind flayer)
In conclusion Hop would be important in the series, and Billy (if he actually talked to his sister and knew what was going on) he would also be important. But on the other hand, he kinda wouldn’t also with the fact he doesn’t want to talk to Max, it would be a 25% chance he would talk to Max, and a 75% chance he won’t.
Sources: Fandom Wiki How search: Jim Hopper and Billy Hargrove.