Welcome back Tenno! I would prioritize getting whatever you need to do all the quests that came out and finishing them first, that will unlock all of the new content locked behind them. From then on, pick a thing that seems interesting and go at it! Zariman and it’s grinds are well worth your time, it has really powerful Arcanes and weapons behind it, weapons that will greatly help you tackle the Archon Hunts that are locked behind The New War. Duviri is worth jumping into as well, both the Circuit and the Drifter part of it. You can get even more powerful weapons from there as well as Kullervo, a new frame that is well worth getting (he will be of great help with Steel Path once you get that unlocked).

From what I recall, Deimos was already during the Viral/Heat damage meta which remains the same. Shield Gating was changed, the duration of invincibility now scales with your shields size and the old Decaying Dragon Key style gameplay is now officially supported through a corrupted mod. Think that’s about it, if you got any specific questions go ahead and ask!

Edit: I completely forgot about the companion rework, they are all immortal now with a Djinn-style revive timer. There are mods available from the Open World pet-themed NPCs that can provide synergies for both the companion and you and reduce the revive timer by performing certain actions. We haven’t gotten the 2nd part of the rework yet that involves changing companions actual abilities, so a lot of them still aren’t great, but Sentinels are a really strong option now! The best picks are Dethcube, Nautilus, Wyrm Prime and Carrier. The Panzer Vulpaphyla from Deimos remains a very strong option as well. Smeeta with it’s RNG double loot is also good, I just hate it’s random nature so I never use it.