I suppose the you can classify upgrades by how easely you can access the slots you plan on tinkering:
Bottom hatch: easy upgrades would be adding extra ram in the bottom slots, replacing the hdd with an ssd or replacing the odd with an hdd/ssd(if you need any of those).
Disassembling most of the laptop to access the motherboard: harder to do upgrades would be replacing/adding ram in the inner slots (if you aim for 32Gb for example), adding an msata ssd or replacing the wireless card with a proper one (i know i plan on doing that at some point). (this might void your warranty tho - far as i know removing the heat sinks off the cpu/gpu will void it, but im not sure how ok is asus with disassembling most of the laptop).
Also a popular "upgrade" when disassembling the laptop to acces the MB would be replacing the thermal paste on the cpu/gpu (i also plan on doing that at some point) - this will surely void your warranty tho.
Thats about all i can think of.