Ehm... Savage, EX etc...
No.

Normal modes with the Trust system yes.
They did it in EW with one of them and I thought it was good for the story.
I also like the idea of just being able to insta hop in and do the fight for fun too.

With that said I actually do think that they should have some kind of a training mode for Savage and EX.
Both because there might be parts I want to practice myself but also because holy *insert a billion curse words here* I am so sick and tired of people failing the easiest mechanics ever and there would be no excuse if people could practice them by themselves.
The numbers in second circle in particular, I have no idea how so many countless people could fail so hard at that in clear or post third arrows groups.
It was one of the most infuriating experiences ever and it rly made me wish there was a mode where people could practice it alone because maybe that'd decrease the amount of people failing so hard at it.

There shouldn't be any rewards for it, but just a mode where you could pick a mechanic and the boss wouldn't one-shot you ( except for that mechanic ) and you had npc's with you doing it too so you could practice.
The npc's shouldn't just always do it 100% correct tho they should ultimately do it correctly but it could be a bit random and some of them could fumble a bit before and only arrive at the correct location in the last second to better simulate real players.
Otherwise people would just know to run to where the npc's are/ aren't.