Gribbles is the end of easy exp in HA's where you just kill down a path and win. I don't think the developers plan to put another Gribbles in the game. HA's beyond Gribbles tend to be things you do to try for certain loot, tokens for loot or to finish progressions.
Most people move to open zone grinding after Gribbles, though you could work your way through TDS or do TBM to build the TBM-token armor sets. The common strat is to grind at the highest expansion your group can survive until you hit 106, buy Conflag for at least your tank and then move to FM/Lcea through 108 (finish all AA up to 108.) Then move to OT for the 109-110 push. OT's southern area is a little easier than the north, less DoTs and snares.
ETWK - A couple of the other early missions are fun to run if you need a break, but Gribbles is the only super fast and easy one. The other ones in Dead Hills and the Bixie ones seem popular.
Darkened Sea - I've unlocked and done the first Heroic, then stopped running these. Too much questing to unlock all the zones and missions for me.
TBM - I did the mirror reward mission, then skipped this expansions HAs. I does have a lot of HAs though. I do know other players used to farm these for the tradable tokens back when they were still worth something.
EoK/RoS Heroics are little watered-down versions of raids so they all have some mechanics to deal with. I've boxed all of them except the Labs one, lost that a few times and gave up. These are not things I ran for xp though, was either to backflag friends or for my own progression.
EoK - The Lcea one is the easiest I guess, but still has some running around, ambushes then helping city guards deal with large waves of Sarnak to win. EoK missions are nice for the rewards though. There are a series of HA only EoK augs that can be the random drop in your chest at the end.
RoS - Fell Foilage in OT is the only one that's "easy." It's a DPS race against one boss that spawns adds, but if you don't have the DPS yet it is a no-go. Once you have the DPS, you can finish it in 10 mins or less per run. I don't know that it is great XP though, I was running it at max AA already. RoS missions rewards are also nice. Chest at end has a random selection of three or four of 106-110 spell runes, armor and loot from the zone you're in. So the Goro HA (quest guy is in Skyfire) actually gives out RoS t2 armor and higher lvl spell runes for wins.
I have done it few times, it's not hard. But I don't like Droga mobs with their annoying spin, so for currency, I ended up farming Praetor Gilfol's (Gates of Kor-Sha) : Mysteries of Kor-Sha and Infiltration of Kor-Sha.