If the FoG expansion is Cabal based and we're getting Mars stuff with the Vex after TTK, I'd really like to see Cabal on Mercury. It could tie into the Cabal that disappeared from Vertigo, add a Patrol area to Mercury and introduce new variations on the enemies. Say that Osiris is able to create this artifact of incredible light but it's stolen by the Vex. When the Cabal group are absorbed/captured by the Vex, they're able to harness the artifacts power since they fear the Darkness as much as the Traveler does. So you could be fighting Cabal that have been living inside of the Vex network and surviving against them for centuries, maybe even harvesting their remains to make new armour, like Kabr. They could also have crazy variations of the Guardians powers. Say that signal that the Skyburners sent out was also received by this Cabal group and they are able to use it to predict their empires attack. Maybe they start trying to build a device that could either destroy the Traveler or the whole system from within Mercury, like a Sun Eater or something. Mercury would also be a really interesting Patrol area since it could be largely internal but very mechanical. There could be moving/transforming platforms and rooms that rearrange their locations on the map or parts where gravity is inverted. There could even by a Strike/mission where you could team up with that Sunbreaker cult from TTK. Imagine fighting a giant Cabal that's also a Sunbreaker. Also a Mercury Raid where at some point you have to wonder along the surface of the planet while it's being hit by a Solar Flare and that's what you have to avoid being wiped by. I'm just blue skying here and I'll admit, this is very unlikely. But it'd be kind of cool.