That's a nice idea. And instead of a portal opening up, the arena itself could change with an animation just like in O4s, O8s and O12s for each boss (eliminates the need for loading time for each member). This can be a part of the achievements that you can do, since currently all achievements are either Level up X class, complete objective X, or do objective X for Y times. There's nothing really challenging unless the content itself is challenging. The only thing there that can really be considered an achievement is doing PotD or HoH solo.

They could spice up the achievements by adding stuff that required you to kill a certain boss under a certain time limit or with a single healer (I was in a party that almost had O10s with one healer lol), defeat Rathalos with all DPS party (They all ready did it with 4 DRGs and I had a party that almost managed to beat him in phase 2 with all RDM party before 4.4 release day).