If you have yet to get into the open worlds, you should definitely start upping your standing with all of those factions. There’s some key items to get (like Helminth from the Entrati and Necramech from the Necraloid) and a few warframes and weapons made available through them.

I’ve heard good things about the Duviri circuit too, but I have yet to dive into that myself. Seems like a good way to get to what would be “endgame” content fairly easily.

I just came back from a long hiatus a month or so back, and also quit around the same time you did, so I feel your pain. This might not be 100% accurate, but this is how I see the various open world grinds from my own experience:

EARTH:

OSTRON: Obsolete for the most part, unless you really want a Zaw. Grinding this will give you baits to get fish that some later recipes need, but for the most part you’re here for parts for a modular melee weapon called a Zaw. These used to be top of the line, but now are just pretty good.

QUILLS: Mostly obsolete. Not sure how far along you are in the quests, but The War Within will net you a new equipment slot for a modular weapon called an Amp. The default amp is hilarious garbage, and makes certain quests paaaaaainful to complete. However, the first rank of the Quills will net you the parts to make an amp good enough to deal with end game content so build that ASAP. Beyond that first rank though, a later Syndicate will give you better amp parts.

VENUS:

SOLARIS UNITED: Gives you parts to make modular primary and secondary weapons called Kitguns, as well as modular Moa companions. Neither is top of the line anymore, but kitguns at least are still a solid A tier in endgame content. You also need to grind this to max to open up Vox Solaris, so might as well!

VOX SOLARIS: Better amp parts. Still haven’t started this grind yet myself, but that’s basically it.

DEIMOS:

ENTRATI: Gives you more Kitgun parts, infested companions, and some good endgame weapons. Vulpaphylas (Infested kavats) are currently the meta, as their special ability is to self resurrect infinitely. Also gives you access to the Helminth halfway through, which allows you grind up old warframes into upgrade materials. Hope you haven’t deleted too many to save slots! You’ll naturally also grind rep while grinding for your first Necromech, which you’ll need to unlock the last chunk of the game.

NECROLOID: Alternative necromech parts and guns. The default setup is really good, so I haven’t felt the need to grind this at all, but you do you.

DUVIRI: High end melee weapons, and Incarnon Adapters, which are upgrade packs for older, powercrept guns (Soma Prime, my beloved…). Incarnon-upgraded weapons are currently top tier, but to use them you need access to the final locked solar system zone, which is again locked behind having a Necromech.

[ZONE NAME REDACTED FOR SPOILERS]

HOLDFASTS: Top tier weapons and arcanes. All but mandatory once you have access, but very quick to grind.

Eidolons and amps are a good next step if you haven’t done them yet. Helminth is a fun system that I love but is more a quality of life improvement for warframes that have abilities you don’t use rather than a strict upgrade like upgrading your amp, or getting better arcanes.

The open worlds for sure, either focus on getting an amp or Helminth. Or you can split it about so you don’t burn out grinding one thing too much. I think amp might help the most in terms of upgrading one system a lot, Helminth is a big resource sink and while it helps some frames a ton, some of the frames you’ll want to use there are locked behind open worlds anyway.

I just came back recently after a long hiatus as well. It’s pretty overwhelming. It looks like I have ti change my builds specially melee. I’m also interested in the answers.

Open worlds factions have some of the most powercrept features released yet. The Zariman Incarnon weapons are just the most blatant powercreep weapons released. Crit 2.0 on all the weapons. 50% crit chance with a 20x critical damage multiplier (technically)