They are dead :(
And there are no tangible plans of reviving them or bringing in new raids/trials.
Endgame is more of a self-defined goal than ever before.
Endurance runs got easier thanks to powercreeps, enemy scaling nerfs, shield gating and other stuff.
Raids are no more and those communities largely broke up. Arcanes are now farmed by killing Eidolons during the night on PoE, they require some teamwork but even public they are kinda hard to fail and are still faster than the old trials. You need your operator to remove their shields, but after you killed/leeched a few rounds you can buy amps (operator weapons) to increase their dmg and operator arcanes to buff the operator. The focus tree also buffs them a bit.
The "new" space combat where you have spaceship (after completing the rising tide quest) and the archwing to fight in space and finish some "ground" objectives to finish the mission.
The grineer missions are often okay, but the corpus versions are really tedious and your railjack really is just a glorified space taxi. So, once you have grinded up everything in space you avoid doing those missions, since the time invested mostly isn't worth the effort. Mech Suits: aka Necramechs
According to the lore they were the warframes predecessors to fight the sentients. The 2 versions we have stamina and are clunky, slow, too big for most tileset and over-all not-fun to play, but the "Voidrig" exalted gun has a ridiculous amount of dmg, so it's nice for some fights. (You get them buy grinding up the Deimos syndicate)
Mods with random stats that only fit into the weapon listed in its name. The stats value are random and vary to a certain degree, but they are mostly based on a weapons popularity (more popular weapon = worse stats on the riven). Those stats are adjusted each new prime access. You can also reroll a riven for new effects and stats (but not for another dispo/capacity/weapon) by using kuva. Kuva can be obtained mostly by doing Kuva siphon missions/or missions on the Kuva fortress, are a possible sortie reward or are part of the nightwave shop or some other activities.
Needed to re-roll the rivens for new stats. It's overall just a gamble game to min-max some weapons. Good riven rolls for some weapons are "worth" some hundred to thousand platinum.
Are kinda mini-bosses you can create for yourself and the "final bossfight" requires you to have a RJ. They have upgraded/different versions of some of the grineer/corpus weapons. Those weapons also have a bonus dmg element depending on which frame you used to create them. Those values are between 25%-60% and you can increase the values up to 60% by farming more weapons of the same type and fuse them (if you have different elements you can choose which one you want to keep during the fusion). These weapons also get +2 in maximum rank for each forma (capping at rank 40 for 5 forma) and each bonus rank also gives mr exp.
The liches also have a 20% to have an ephemera matching their element. (ephemeras are a fashion/custom aura for your warframe)
If you only want the mr you can stop once you have all weapons and maxed them out with 5 forma and never touch liches again, or you try maxing out the 60% bonus dmg too. (Only did that for the weapons I like).
Kinda like the corrupted mods from the dragon vaults. They have slightly worse stats than the normal versions, but have an additional bonus. These mods have fixed stat's that won´t change. There are some amalgam mods for 5 weapons (dropped by a new jupiter boss called "Ropalolyst") and 4 "universal"/weapon class ones you can get once by doing the thermia fissures events every few weeks.
Replaced the normal alerts. It's like a battle pass but without the premium bonus/bonus loot. You get a new task every day/week which requires you to do some ingame stuff, some of which you'll do by just playing the game, some require some specific farming. As reward you get standing and rank up. Ranking up gets you some loot directly or some currency to by the "old alert" rewards, cosmetic, nitain, potatos, kuva, aura mods, etc.
Got released just a few weeks ago. Base stats are slightly worse than their normal versions, but they stack their effect (often via getting kills), thus they surpass the normal versions pretty easy and soon. Imo totally worth it. You can buy them with the arbitration currency via the arbiter of hexis.
Arbitrations are kinda like the old alerts and you have one every hour. In this mission you get a +300% boost for a random frame and a weapon (changes together with the mission every full hour). If you die in these missions' teammates have to revive you by collecting 5 shards dropped by drone enemies, you can't revive yourself in there.
(Corrupted Holokey short:) Holokey:
Holokeys are a rotation reward for opening relics in the slow RJ fissure missions. You need them to buy 4 corpus weapons from the perrin sequence. These weapons are kinda like the corpus lich weapons as where they have random bonus elements, changing every 4 (or5?) days and you can fuse them up to 60%, too.
You need at least 160 if you want the mr exp for all 4 weapons.
DE is currently looking into their drop volume and if they change for a better one. So you might want to skip them for now.
Needed to kill your Lich. You get them by opening requiem relics which you can get by completing kuva siphon missions or killing the liches thralls in lich missions.
After that you equip them into your parazon (finisher weapon). After that it's like a game of master mind in which you have to guess which 3 mods out of the 12(13) available are the right one to kill you lich. You test the mods by facing off versus your lich and stabbing them with the parazon. After a few stabs you should know the right ones/order and you can kill the lich in a RJ mini-boss rush.
Kuva (grineer) and Tenet(corpus) weapons are the lich variants of the normal weapons. they have a bonus element up to 60%, different stats compared to their normal counterpart and can be ranked up to rank 40 with additional forma.
A skill tree for your RJ (and AW).
Unlocks bonus abilities/skills for different "roles" on the RJ, like teleporting in RJ missions, more AW speed (in all missions with an AW), faster RJ repair, etc.
Each rank also gives mr exp.
I would guess most are considering "the steel path" as new "endgame". After finishing each node on the starchart at least once you can visit Teshin and get a toggle for the starchart where you can replay the starchart with slightly buffed enemies (+100lvl and +250% shields/armour/health) and an inbuild +100% resource/mod drop chance booster. You also get some new arcanes for your primary/secondary weapons here.
For meta: Most likely stuff with AoE and armor strip/dmg buff.
Focus on "special builds/focus on mechanics/focus on fraction" is only needed if you want to do very long runs, most of the normal starchart stuff can still be done with anything.
Current "endgame" is mostly endurance runs and fashion. There is nearly nothing requiring a premade squad/good coordination or a competitive clan/ally.
Last meta was heavily focused on melee fighting, but with the latest mainline update (melee nerfs/gun buffs via new stuff) the meta might get a bit more mixed.
So I expect most primary/secondary builds to change to a galvanized+gun arcane combo, but to see which are the new meta builds we have to wait a bit more.
For being useful: Anything that helps finishing the mission faster/makes missions more efficient.
Pure support frames like Trinity or pure CC only are kinda rare. AoE/DMG buff/speed frames are the most meta.
Waste of time: Warframe
it's a limited sandbox where the current endgame is defined by yourself
(imo RJ & Liches are a waste of time. You grind the stuff once for the MR/rewards and for the sake having it maxed.
RJ missions are just way to slow/inefficient for farming. And selling Liches (with their weapons/ephemeras) to other players to make some plat isn't really worth it, too)
For what isn't a waste of time: I have no idea.
Currently I play for having fun with the games fast gunplay and missions or I play stuff that looks or feels cool.
And while doing this I try maxing out my profile again. (But this might take some time, since farming Holokeys and the Ambassador weapon are imo not fun.)
Imo you should try unlocking everything on the starchart, finish the open quests and do at least the first few ranks of each syndicate (for Fortuna one: get it to the max rank to have access to another/hidden one). But there is no need to hurry.
This should dip in nearly everything the game currently has to offer.