I assume the 3.0 light subclass reworks will have all elements free to all players, but if not, they should. No tying middle tree to an expansion, no tying Stasis to Beyond Light, which in the past year, has often literally been pay-to-win in PvP. While they may arrive during an expansion, they should be free to all players. To stay “current” with the game, you’d still have to purchase the newest expansion at launch, but this would make catching up much easier with a lot of content continually offered to free players. This year, when Witch Queen launches, Beyond Light would be made free.
When a new expansion is released, all previous expansions are made free.
Destiny 2 changes over to a new “rolling” expansion system.