Making Explorer's League relevant as a faction - Exploration based gameplay.
This faction has been in the game for ages and not much has been done with them so here is my suggestion, make them a faction that rewards players for exploring. Back before cataclysm loads of people wanted to explore unreleased zones/instance and they got punished if they somehow managed to do it, well Blizzard should be encouraging exploration instead of punishing it.
It would be nice if blizz made hidden/secret areas, zones, islands, instances etc. for those of us who want to explore the world they have created and reward us for exploring them. It could work a bit like archaeology but instead of digging up the past in normal zones you are exploring hidden ones and what not.
Give the faction dailies/weeklies that send you of on a kind of treasure hunt to find hidden parts of the world. BRD would be an example of a good place to have a hidden area within an instance, there could be a hidden instance portal somewhere in Un'goro, maybe there's a secret tunnel in Arathi highlands that leads to secret underground city, there could be an island to the north of Pandaria.... the possibilities are limitless.
These secrets could be protected by traps, creatures or even bosses. Some of the rewards could teleport you to these hidden places, there could be a mount based of a unique creature you found on the other side of Azeroth.
The only problem with this suggestion is that there are people who are just plain too good at looking for stuff that would figure it out early on PTR and there would be guides all over the place before the patch goes live.
Sounds cool but WoW is a massive game with gigantic playerbase, so nothing can remain secret for more than few days. This would end up being 'go to wowhead/mmo-champ/icy-veins and look up what you are going to explore beforehand. And if you are not doing that you are gimping yourself. It's just a nature of big mmorpg, there are no secrets here. I already accepted it, because hoping for something like you are proposing will never happen, sad but true.