With a level 25 cap, some high-level quests will be in theory doable, sometimes with a full raid help. We have the Gnomer quest that rewards awesome blue gear, but we also have two bigger things for the Horde:
- The Corrupter quest chain in Desolace
- Test of Lore in 1K Needles
The final quest for The Corrupter requires killing a level 40 Elite Demon (25-level required for the quest chain!) so you will need a raid helping you kill your tag 
The reward is a good shield or mail shoulders you will be able to use:
https://www.wowhead.com/classic/quest=1488/the-corrupter
Test of Lore will require SM Library as well as killing higher-level elites in 1K Needles. The final quest gives the best DPS wand for level 25 or a 1h mace:
https://www.wowhead.com/classic/quest=1394/final-passage
And for late night grinds priests and locks and some other classes will likely want to farm RFK mobs for BoE that are 24-25 for the most part, like super-BiS Shadow spellpower staff. RFK minimal level to enter is 25 Similar to some SM dungeons.
As Gnomer and SM are expected to get their raid versions it’s unsure if we will be able to enter normal instances to get the quests done now… time will tell.