I actually like Nibelsnarf, it's a prime example of how to make a Cephalos replacement interesting to fight, except they forgot to actually take Cephalos out of the game.
Speaking of that, I hope in future games Capcom starts ditching older monsters in favor of better ones, or at least update the older monsters with better animations etc. They have greatly improved on monster design and animation since their earlier days and it's rather jarring having old monsters in with much more polished ones. I don't understand why they do things like drop Great Jaggi/Wrogi/Baggi in favor of the raptor dromes. Great Jaggi is the penultimate newbie punching bag, even better as a tutorial monster than the Great Maccau IMO and leagues better than Velocidrome. Great Maccau is still a pretty cool beginner monster, but not "first monster you fight" beginner.
The most interesting thing about Khezu is the lack of music to add a bit of creep factor, but I remember Gigginox being a much more interesting fight... although it may just be my nostalgia since 3U was my first game. I heard from the internet somewhere (what a great source) that apparently the Japanese audience loves Khezu, so Capcom chose Khezu over bringing Gigginox back.
Qurupeco is a more interesting Kut-Ku with a gimmick that also teaches players to start carrying dung bombs.
Barioth is sort of a slower Nargacuga but he still has unique attacks, yet nobody seems to remember or care for him because he's not a cat and Japan is obsessed with cats...?
Even Rathalos could use a bit of polish and he's the series' flagship monster. Currently he can be hovering in the air casually, and the next second his tail is in your face with no warning whatsoever. His wind-up animation doesn't even have to be long, just combine it with his ability to turn in midair and it'll make the attack appear much more fluid (and even come out faster since he doesn't have to spend 3 seconds turning towards you). It would be better than no windup whatsoever. Yes, I get that you're not supposed to be standing in front of him when he's hovering, but you have to agree that the way it animates is pretty bad to look at. Then of course there's the issue with the homing claws, that move is just wrong. Considering he has the air-charge attack which is fairly slow despite looking like he should be flying at you full speed, the homing claw attack can potentially travel twice as quickly even though it doesn't at all look like he's charging at you.
It's not like they're afraid to do any changes to monsters, etiher. Just look at Lagiacrus.