A trap for a Titan.
It's still a hottest point of debates. From one point of view, Sargeras is being heavily up as an endboss, spotlighted, told a lot about, etc. All other major figures of Legion showed off, fought, killed. So he's a climax, any other turn would drastically break down the tension. No other viable target for things as mighty at the Artifacts to be drained.
On the other hand is what you all know well: BIGGAH DUN PLENIT!!!1.
Okay. First, just to focus a standpoint: IF that reason be valid reason that he can't fight him now, it will NEVER be possible to fight him. At all. Because at no circumstance we ever will grow to size as big as continents, okay?
If he's bossed not in Legion, that's 2 ways - he pops out of nowhere at some later xpac, or another xpac where he is footaged again. Just imagine in WotLK we destroy Scourge and its generals like Putricide, who is an expansion boss (so cool, yeah?), then Arthas kills us all and goes somewhere away. After all the quests and books and other focus he had, lol. Only to appear several expansions later to steal our kill on Kil'Jaeden or Azshara and become a SUDDEN SURPRISE ENDBOSS! Or just reiterate Scourge assault again, but we are strongah now, so we beat him. Both is extremely, dramatically stupid and moronian variants. But people are ripping their asses inside out to write crap like this, only to deny Legion the endboss it's being built up to since beginning, every bit of our epic struggle to bring any bit of power and any allies, EVEN Lich King, Old Gods and Elemental Lords, all to gather power sufficient to match the Dark Titan.
But yeah, I remember, biggah dun plenit.
Let's make a very likely proposition, which is supported by various facts.
Sargeras CAN manipulate his size and shrink himself. Without losing his power, too.
Evidence to the latter: Archimonde in HFC, Hyjal etc is TIMES larger than Kil'Jaeden in ToS, latter is quite tiny compared to Archi, but it's obvious that it no way diminishes his magical power.
So, Sargeras, too, can manipulate his size at will. I even have a hypothesis - in his "planet-size" form he is comprised of more ethereal matter, like stardust, titans likely use this form to travel. He surely can utilize it to fight and manipulate his energy, e.g. to project a blade to slice some watermellons, planets etc.
BUT.
He likely can shrink and appear just BIG, like Ragnaros, or Avatar, or any. Reasons: on his first crusade, he FOUGHT demons, not WIPED them like we wipe dust from cupboard with a rag. He might had a reason - not to wipe all being on the planets he wisited, but specifically demons. But he had to land in full powers, because there were hordes and lots of them, if he projected only avatar, they would destroy them with numbers one by one until he would be drained out of powers. Not an option.
Secondly, he prepared to enter Azeroth during WotA through a portal in the Well. Well is a lake, not a hole of entrance for a planet-sized creature.
But amounts of energy at work was unparalleled, not avatar-sized. So he was entering in full power, but in comprehensible size. And Brox actually scratched his leg. You can't scratch a leg of a planet.
Arguably, he also personnally fought Toranaar, King of the Aldrachi, and the latter wounded him in the chest, bleeding the titan. Yes, nathrezim write that could POSSIBLY be an avatar. But it was not stated as a fact, and I doubt it can be the case. Reasons: firstly, powers at work. Even regular Aldrachi are describe supremely powerful, one warrior at cost of hundreds of demons. They were WAY above us, Azerothians. Yet, our champion, Aegwynn, was able to defeat the Avatar, of which Sargaras never cared a bit.
Yet it's unlikely that king of Aldrachi was weaker that he could to nothing to the Avatar. If he's weaker that Avatar, Archimonde would pwn him, too, and then it's no reason for Sarg to take any action. So likely Toranaar rpwned Archimonde, that was unique and thus demanded Sargeras' attention. So I think Sargeras actually came in his full power, toyed with Toranaar for days of fighting, but king tricked him and dealt a blow, wounding Sarg, who was ENRAGED. Yet he never shown ANY concern or emotion over losing Avatar to Aegwynn. Looks like he was scared that even HE himself, can be wounded by other than his kin.
So, my statement: Sargeras can shrink his size while traveling through the Twisting Nether and arrive on planets through portals. It this state, he is just as strong in terms of magic, as he usually is.
What is his exact power? Simply, it is equal to a sum of magic of a whole planet. Which is immense and not comparable to any single creature, but still has a limit, planets can be broken. It's much harder to break a titan than a planet, because he, too, can use his power against you.
Now, why he will arrive at Argus in shrinked size? I see 2 reasons. First - he doesn't want to destroy his main base only to kill us, invaders, for same reasons as before, just reversed sides.
Second: he is physically far away from Argus, but can easily access it via Nether, through portal, which requires smaller size to enter. Likely the case, first thing Kil'Jaeden must've done is to build a stable portal on Argus for his master to activate and enter in urgent cases quickly, when necessary.
And he's not a bit feared to appear being small, because his power is still the same, we are no threat in his eyes.
So finally, HOW do we defeat him? So, we will be able to technilally fight him. But his power is equal to magical energy of a planet. Simply - we need same. And we can have it if we blow Argus AND focus all the energy of its explosion not just outwards, but focused at a spot, at him.
So, Illidan has already made needed calculations, I guess.
But some power will still be lost, so we need more.
Here go ARTIFACTS. They are all powerful. Warblades once actually pierced his skin and bleeded him. Sataiel planned to use Ulthalesh on him, considering to rival him one day.
Scepter was strong enough to shatter Draenor. And so on.
BUT HE KILLED WHOLE PANTHEON!
Just read more carefully, okay? He was backed up by thousands of demons to distract titans. AND, they, as pure Arcane, had EXTREME vulnerability to Fel. Just pressed a great weak spot on them. But Light and possibly Nature are far more resistant to Fel. So, ARMY OF THE LIGHT and NAARU in person will aid us on Argus, and so we can withstand his attacks. Then we use artifacts to their full power to wound him, break some defences, EXACTLY like we did with Deathwing and his plates, then kite him to a designated spot, wounded and armor exposed, for the focused blow of Argus being imploded, which will vanquish him for good.
So, our mission is EXTREMELY hard. We must repel all the might of Legion on it's own base, cut off opened reinforcements portals, repurpose them to make planet-blowing trap ready, clear his remaining generals in the Palace, do all that before his emergency portal is ready and he enters.
It will take all artifacts powers, all buffs from Keepers, all the Army of the Light and the Naaru only to withstand his assaults and just wound and weaken him a lilttle before he unleashes his full power.
Then he must be kited to a very certain spot. Likely Illidan wants us to somehow not just blow Argus, but to reflect Sargeras' own attack that could break planet, back at him, blowing a planet and adding this expulsion to his own unleashed attack, focusing it right at him. That would take constructing IMMENSE and wery complicated trap, way bigger than for Archimonde or Deathwing. Yet still, power of exploding planet + his reflected own blast on weakened and wounded Sargeras should be enough to tear him apart.
And remember the direct words of Xal'atath: "you must hurry and defeat the fallen titan, there are greater battles yet to fight".