I think prowler and Arts should definitely stay!
That being said, what they need to work on is - balance.
Hunters are more versatile and stronger than ever, and somehow the monsters feel weaker than ever.
I understand that you can hunt by yourself, or only with one more hunter, but I would argue that this game is at its best with 4 players online. At the very least, that is what I enjoy the most.
The problem there, is that doing so, feels like animal abuse. The monster die way too quickly and nothing is a challenge - not deviants, not hyper.
I started with MH4U, and I felt monster lasted way longer there, and G rank made the game last much longer as well.
Problems I see -
1) Monster's health.
2) 4 player hunter balance
3) No G rank, longevity- for some reason I felt I reached end game way faster than 4UH
4) Getting necessary items seems way more easier with the trader, like with armor spheres. Feels like it takes no effort.
Simple things that I think are easy to solve.
The new arts are awesome, and I love the changes to the glaive upgrade system!