The "1999" expansion coming up seemed promising, 
I've always known Warframe is like an asteroid belt of content updates where each asteroid floats somewhat independently of one another but trying to put it together is actually discouraging me from diving in.
I see stuff that sounds very cool, like some new fancy way of upgrading my old Bo Prime to be a stat stick of increased sprint and bullet jump, but then its like "oh you have to go to yet another grindy hub and collect a lot of stuff and probably do daily leveling tasks with local rep" which was what eventually made me set the game aside; during The New War that part with the mandatory Necramech sequence was such a story-momentum-stopping bottleneck (because I didn't have a Necramech and I didn't do the grinds necessary to get one because I hated that particularly goopy and unpleasant hub area) that I paid some real money, got a quick and dirty Necramech unlock, and only then completed it, felt tired... and logged out for years.
Also, as silly as it may sound, the notion of "feeding" my old Warframes to the Helminth to min-max my action bar felt so whiplash-like counter-intuitive to everything my Operator's story had built up to at that point that the decision to stop logging in was partially pushed by that once that was the new hotness content.
I remembered this resonating with me:
"...We had created monsters we couldn't control. We drugged them, tortured them, eviscerated them... We brutalized their minds... but it did not work, until they came... And it was not their force of will, Not their void devilry, not their alien darkness... It was something else. It was that somehow, from within the derelict-horror, they had learned a way to see inside an ugly, broken thing... And take away its pain.''
And now it's "ah naw, go get fed into the talking tumor, I want a better action bar." 
I guess my question is this: where does an old veteran even start with the avalanche of new content? Is it all rep grinds from content asteroid to content asteroid from here on? As silly as it may be to ask: is it still fun?
I've been on a break for a couple months myself to play other shit, nothing negative about the game itself because it's honestly more fun than it's ever been. The design director position was handed over from Steve to Reb and there's been a fuckload of quality-of-life improvements that feel like they're coming from people who love playing the game as much as they love making it. It's still a content archipelago as is the curse of the live service game, but the mission variety has kept improving, enemy design and variety keep improving, new frames and weapons are good fun, buildcraft options are through the roof.
My sense of time is beyond fucked, but I think your next things to check out after The New War are the 3 quests Angels of the Zariman, The Duviri Paradox, and Whispers in the Walls, which all lead to new mission hubs. See if anything catches your interest and branch out from there.