Not that much. Yes, AoE2 usually adds new factions, that won’t happen in StarCraft II. But introducing new units or reworking existing one is possible.
Adding singleplayer mission is pretty mich the same.
Also the Co-op mode of SC2 is quite popular and there is room to add a “new factions” there.
No they’re VERY different from what I checked. I’m not sure how you could possibly say “not that much” to that!