Here's an idea (regarding armor upgrades) (Destiny)

by CyberKN , Oh no, Destiny 2 is bad, Wednesday, February 04, 2015, 16:30 (3377 days ago) @ dogcow

I think the gist of the proposal is that you can use any piece of legendary armour to hit max level. Raid-themed armour still drops, and has perks that make it beneficial to use during raids, but its default stats aren't superior to other armour.

Presumably, each time a new expansion comes out, three new upgrade nodes (or something similar) are added to allow you to keep upgrading your armour to reach whatever the new Light cap is, and those nodes will require some form of Radiant shards.