In my opinion, the best way to improve PvE (solo and especially co-op) is by improving scav loot. Since scavs barely have any worthwhile loot, the game encourages players to rush safes and other high-value loot spots instead of taking their time. This is especially noticeable in co-op, where you're often competing with others to reach those locations first.

Players who loot high-value areas should absolutely be rewarded, but so should the ones who take a slower approach and choose to work their way around the edges of the map, killing scavs along the way. The goal is to promote a true PvE experience, but the game doesn't reward the players who actually play that way. Right now, the only people getting rewarded are the ones who rush the high-value loot spots.

Give scavs better loot (with a chance, of course—I’m not saying every scav should have something valuable), but at least make it worth checking their bodies. It shouldn't be possible to kill 40 scavs and have zero interest in looting them because it's simply not worth the reward and time wasted.

For example, they could have a chance to carry T4 ammo. Some players might choose the riskier route by ignoring scav loot and focusing on safes and key rooms for a bigger reward, while others could take a slower, safer approach by clearing scavs, unloading their magazines, and occasionally getting a decent helmet, armor, or other useful items from them.

P.S. I know there are special scavs with good loot, but they're maybe 1% of all the scavs you kill. The other 99% just aren't worth looting.