Yeah- that's a pretty big deal. Pre-2.0 everyone was very accepting of change because the state of the game was pretty poor. Nowadays, we're in a very good state, with very a matured metagame. Adding 10 levels wouldn't rid us of Greater Rifts, etc, but it would definitely wipe out our precious gear that we've been grinding for since 2.0 was released. And I think the game has grown beyond 'gain 10 more levels to grow in power' to a point where, get certain gear to grow in power. I don't see any benefit in changing our cap from 70 to 80, it's entirely irrelevant.
Unless it is their intention to push the reset button on currently available gear. Who knows, maybe their hands are tied on creating cool new legendaries simply because of too many existing items that cause conflicts.
On the otherhand, the adv mode content from ROS was awesome, and it would suck to not get a new Act and set of bosses. A decent reason to raise the cap to 80 so the final boss isn't immediately killable.
With the introduction of seasons I don't think the shift to a higher level cap would be that hard to swallow. The ROS shift was before we had seasons and there was still the remnants of the auction house to contend with. I know lots of people have geared up non-season, but I would guess that most players like to start fresh each new season, so I could see them coinciding a new expansion with a new season basically encouraging everyone to start playing back at level 1.
That said, keeping it at 70 would be fine too, honestly.
Regarding a new act I'd like it just for the monsters/zones/bounties, and to play through once just for kicks. Even if the story is just an excuse to shove new zones/monsters into the existing randomized content, I'd be fine with that.
Hoping to see:
*One new set for all the classes and rebalancing for the sets that are obviously weak. Hope they don't abandon buffing/reworking old sets simply because 2.4's main focus was on this. There are some very fun but also pretty bad sets out there like Wrath of Wastes for Barbs, Shadow's/Firebird's for DH's/Wizards (these two unfortunately ate too many nerfs on the ptr), and pretty much every 6 piece for monks lol.
*Some sort of endgame content outside of GR's similar to PoE's raid dungeons/bosses. Would be awesome to get some very challenging set bosses/dungeons with a specific reward or loot table.
*Legendary affixes for all the items that remain without one
*Crush the 3 support + 1 fotm DPS group meta that has been plaguing every season. I don't know how, but it would be nice to get some more multi-dps variety in there.
*New legendary gems. It's about time we got a fire gem now that fire isn't the end all be all element.
My dream 2.5, would be nice just to get 2-3 of these though. Man, I really want to see a new diablo expansion announced soon though, this game is still missing so many awesome class archetypes: Rogue (melee assassin), Druid (Shapeshifter), and Necromancer are what I want to see.
I don't know what the POE raids are like, but that's kind of what they tried to do with the infernal machine zones. The problem is that those bosses quickly turned into a joke because the power level of gear has risen so much in the last few patches.
I do agree that having a few more "things" to do would be awesome. The bounties/rift/grift loop is pretty good at the moment, but it does get a bit tiresome once you're at the "endgame" (which, granted, takes a while to get to. I'm pretty close to that point this season). Once geared you are either completely breezing through TX, or pushing GRs to the nth degree. I would welcome a few more activities if they could make them feel worthwhile to do to avoid them becoming ghost towns. No idea how those would fit in to the current game environment but I think they could find a way.
And I don't think anyone would argue against wanting a new class. Definitely expansion material (just like Crusader was, of course), but that would be awesome.