Earlier I started asking people who were in Emps-World cc what kind of skillcape perks they'd like to see and few players came up with some.
I wrote them down and compiled them into a list:
https://pastebin.com/raw/x7wKDARE You can see notes at the top as well.
Here's the list of perks on forums as well: (Note that it's copied from pastebin so formatting is bad, also highly recommend looking at the pastebin instead as it contains a few important notes.)
One note I mention here as well is that perks could and maybe should be something that helps you to train towards master skillcape and 250m.
attack:
strength:
defence:
range:
hitpoints:
pray: acts as both burners lit in POH
magic: swap spellbook anywhere (needs a cooldown, interesting in pvp scenarios)
runecraft: pouches don't degreade
construct:
agility:
herblore:
thiev: you won't get stunned by NPCs you're able to pickpocket
crafting: when you tan hides you sometimes tan extra and they're sent to bank(?)
fletching:
slayer: ability to teleport to all slayer masters
hunter: Increased number of maximum traps that can be set up.
mining: 3 ores instead of 2 (might be too much?), chance to instantly mine the rock
smithing:
fishing: increased chance to get casket and snapegrass, invisible +5 fishing level so you can catch manta rays faster (same as fish pot)
cooking: never burn any food, chance to cook faster
firemaking: light fires faster
woodcut.: increase chance to get bird's nest, need something else as well probably
farming: chance to instantly harvest the whole patch (herbs, watermelons, limpwurt roots) OR less growth stages when growing trees?
Feel free to post more of them. Skillcape perks seem to have been highly suggested and wanted update.