Great thread, with a lot of interesting ideas.
The first thing to say is that no one is stating what game difficulty you're playing on. I'm doing my first run thru on Sword & Story, which I took to be 'Normal', & assume that's how they imagine most players will approach it. I'm Lvl 26 now & starting to feel like it's getting a bit easy for me.
I'm not sure I fully understand the whole scaling thing, or how & where it might apply. I think that the problem lies elsewhere.
Much as I like & admire CDPR, & believe them to be about the best story tellers in gaming, I think it's the plot of the game that's at fault.
We start as a mature Geralt, with his memory restored, so we shouldn't really have any trouble dispatching bandits, drowners etc., especially as there's no plot reason for him not having good gear equipped. There, it should be their numbers, tactics & equipment that make the difficulty.
The whole 'open world' thing is part of the problem too. If, by wandering around, you can encounter non boss monsters that 1 hit kill you, that's a game design fault.
It would make more sense if you couldn't access certain areas until you'd done certain main story quests, thus being the appropriate Lvl for the upcoming encounters. On the side quest/non boss enemies front, a similar trick could be used, whereby you can't get through a heavily defended/gated pass, for instance, until you earn the locals favour by doing enough side quests for them & gaining a friendly/admired status with them so they let you thru.
I agree with the Skyrim style skills upgrade by use, just needs to be carefully calibrated. The current 12 skill slots limit is good I think.
Lastly, the worst element of W3 is the treasure gear. When you've invested a lot of time & effort into getting a treasure hoard, it should be pretty good gear & stay pretty good gear! I like the weapon/armor diagram thing, coz this means that the game could control it's leveling by restricting access to various smiths of differing skills by either of the methods I've previously mentioned. They've really missed a trick there I feel. Example, we go thru 2 pretty high level quest to afford/allow 'master' status to 2 smiths, only to find that when they craft a diagram, there's no stat difference in it to a 'journeyman' crafting it! If they controlled access to various areas, they could have allowed for Dwarven & Elven smiths to be encountered later, to further boost weapon/armor stats.
