I didn't think it took long at all to upgrade weapons in D1. In my system, you wouldn't HAVE to use it to upgrade, you could also use cores like you do now to skip that process. I'd honesty love to see cores go away completely as well, but there's a good many that don't want them to go away so I'm trying to find good middle ground here. Using a weapon to upgrade would still need some working out, like how long do you have to use it and what would you need to do with it. Kills obviously to further progress and multikills and precision kills should progress it even further. In any case I wouldn't want it to be some insane grind like for a weapon quest. It should be simple and a fairly quick process.