As for scavenging for materials, looking for them in standart WM encounter should stay limited to basic resources and only very small chance for HQ ones to get in standart encounter. Even with removed possibility to constantly jump in one square to farm them, the activity is not that entertaining.
If you would like to make obtaining materials fun, I recomend idea of scavange in fixed areas, where you would have much bigger chance to meet some players. Lets say they are numerous, with maybe guarded entrances, with a lot of submaps. There would be barrells with their own cooldown that player could try to scavange. Results would wary, they could be tied with skills, gathering standart materials in there wouldnt increase your CD as much as in normal encounter, there could be some more or less dangerous NPCs etc. Options are numerous.
So in conlusion:
0.Players would still have normal cumulative cooldown.
1.Normal materials, like fibers, fruits, wood, still avaiable in random encounters and in fixed locations like Modoc and Waterworks.
2.There could be a chance to meet some HQ materials at random, like there is a chance to meet car with FCC.
3.There would still be fixed locations like HQ mines.
4.There would Scavange locations, lets 8 maps of ruins, connected together like normal town where you could scavenge and find various resources...and meet some players in the process.