Hunt Beta = Ranked, except without a public display of your rank. It’s probably more accurate to say that Hunt Beta is “Hunt” before the map variants got added, but with ranked style monster and hunter queuing (with separate skill ratings for each side)
Arcade = Hunt as it is now with the variants.
There are longer term plans for match making, these are short term changes that are made to tackle some pressing problems with perception of what the modes are.
Currently Ranked ISN’T a ladder, it’s not a ranking system as you’d expect it from other games. It is basically just operating to try and match up like skilled people. If you don’t improve in skill you should, in theory, never move from your skill rating significantly. But, because it’s called “Ranked”, and because there are leaderboards and progression up and down those boards, people think that it is something which it wasn’t intended to be.
So the change to Hunt Beta is to alleviate this misconception, while also providing a space for people that have made it very clear they don’t want to play anything but a “pure” hunt, not variants, and also a space for people that do not want to play monster, or do not want to play hunter. Primarily the change to Hunt Beta is to make it more clear that if you’re the kind of person that wants to only play one role, and wants a slightly more “competitive” or “try hard” game, then Hunt Beta is the one for you.
The change of the name to Arcade is part of this, it makes it more clear that the mode that is currently called Hunt is not meant to be anything other than fun, and sometimes this means silliness as with the map variants.
But like I said, they also have longer term and as yet unannounced plans for match making, this is the first step to clear up some issues that gamers are having right now and hopefully provide a better experience for people by offering much clearer spaces in which they can play.