I mean they sort of already were, even without a titans guidance. That’s precisely why Sargeras and Aggramar were tasked with defeating & sealing them up. 
It’d be curious to see demon armies seeking worlds simply for their own bemusement, hunger or lust for power. 
It was more the titans need to force Order upon everything they came across than the demons being actual threats. Which they clearly weren’t seeing as how Sargeras easily jailed most of them on Marduum and he cleaved a planet in two
He cleaved the planet in two due to the void, not so much the Legion.
However yeah, keep in mind the demons can generally traverse through the nether and travel the great dark beyond to access other worlds and wreak havoc — So much so that they could even lay waste to entire worlds and potentially harm world-souls in the process, hence to why they set out force order upon them … To which, even if the Titans turn out to be our enemies in the future – is a rather understandable & righteous act to take 
The Rusted Legion led by Prince Mecha-Jeraxxus!
Conquer and rock out while doing it.
What are the ranks of peoples and/or demons invading made up of …
Heavy metal demons and mortals.
How did they come to be recognised or reintroduced to the denizens of Azeroth?
Think Ashes of Outland
I’m actually of the opinion that the Legion should never be a threat to the cosmos anymore. A threat to Azeroth, sure… but not the entire cosmos.
I like the idea that without Sargaras at the helm, there’s a power vacuum of Dreadlords, Man’ari, and Pit Lords each vying for power given the chaotic nature of demons. Having demons kind of pop in every now and then as a planet-level threat to Azeroth via some power-hungry warlock summoning them is my preference for the Legion going forward.
they are still immortal creatures, all thats changed is they likely have just balkanized