as much as i think The Witcher's open world is kinda overrated and despite the fact i thought GTA IV was much better than V, both of those games are pretty much void of filler content. every side quest or extra activity you can do explores a new mechanic, has its own story arc, has dedicated voice acting and it's own plot, takes you to a new place in the world, whatever. i think "filler content" is just where it's a repeated activity you've done before and has either been procedurally generated or clearly hasn't had anything "new" put into it. NPC says a stock line, you go and collect something or kill someone, bring it back, done. GTA V and The Witcher's only filler content is going around killing things or seeing shit, but that's not the content the developers have put in, that's just the player fucking around with the mechanics. in TW3 you can't even grind for XP cause you can only earn XP through missions and side quests.