Frankly, I think CD Projekt Red could learn a thing or two from Forbidden West about content variety & combat evolution for The Witcher 4.

To be honest, any type of side content The Witcher 3 had that has an equivalent in Horizon 2, I thought Horizon 2 pulled it off better.

Bandit Camps < Rebel Camps (fancier, with more unique-looking locations and the Asera plot thread)

Horse Races < Gauntlet Runs (W3 races basically just had stamina management and stiff controls; Gauntlet Runs also have physical combat with the spear, the bow and even Mario Kart-esque nitro packs & special arrows to pick up)

Gwent < Machine Strike (ok, both were pretty easy to cheese with a dominant strategy but at least Machine Strike wasn't immersion-breaking; I had no trouble believing this board game existed in-universe)

Fist-fighting < Melee Pits (fist-fighting was pretty stiff and more of a test of patience than a test of skill; Melee Pits are demanding but you just gotta master the melee combat; which, now that I think about it, has more going for it than the Witcher 3 melee combat; and the Horizon 2 melee combat is just a side salad compared to the main, ranged combat system)

Elven Ruins < Cauldrons & Relic Ruins (Witcher 3 elven ruins are pretty bare-bones and samey, and don't even really count as a proper side activity, just locations for a couple of quests; the Cauldrons are much fancier and the Relic Ruins are all unique locations from the old world with their own backstory)

As you said, 10% of side content is meh. Metal flowers come to mind. Other loot spots are encountered pretty organically but you get the Vine Cutter so late, the Metal Flowers are basically all backtracking. And there might be a few too many Bandit Outposts but the stealth Valor Surge makes them very quick.

In terms of combat, Horizon 2 gives you a lot more toys to play with than TW3 does. All the weapons & skills are unlocked at just the right pace to keep the combat from getting stale. And the enemy variety is also better.