Random stats dont work very well in DnD. Itemization in the way it works in OS2 probably wouldnt work without changing the core rules a lot. I would hope for a lot of creative magic items that do unique things.
I personally enjoy throwing magic items at my players that have certain requirements within them. Like a shield that only applis its enhancement bonus as long as the wielder acts chivalrous.
As for ability trees: why? Swordcoast legends tried this and people hated it. 5E might still have a lot of trap options, but its nowhere near as bad as 3.5 was in that regard. As long as you make sure to not fuck up as badly as NWN in which Horse Riding was a skill without horses beeing implemented in any of the official modules, just stick wiht the PnP system. Feats already pretty much work like DnD feats in OS2. I dont see why you would change that.
What i would change is giving all martials access to the battlemaster maneuvers
EDIT: Also give me a blood grapple rule already, for some reason grappling is just forgotten about in all DnD games, depsite it beeing a realy good tool in melee combat and it realy helps to alleviate the problem of "Fighter has nothing else to do than whack the goblin over his head with a large branche"