To me, there are many speculations from you, guys, which actually can be linked with each other, at least imho.
Someone pointed out his resemblance to the Kologarn. Now, who exactly was Kologarn? A Titan? Naaah. A Titan construct? Yep. And he was, as greatly explained up here by Netherspark - a Titan construct designed to guard something, a Keeper. Kologarn's duty was to protect Antechamber from the trespassers. Now, was Thorim a Keeper as well? Along with Mimiron/Freya/Tyr/Hodir? Yes, but they were specific Keepers, with a way greater purpose. They were meant to keep Yogg-Saron in imprisonment. But they were still Keepers, so Watchers are Keepers (Watchers, not watchers, the difference is in a big letter... "Watchers" are THE Watchers, specific group of titanic watchers, aka Keepers) aren't really interchangeable.
Now, we somehow determined that big, clearly defensive Titan constructs are called Keepers and are deployed in order to guard certain places and secrets in it.
We have Kologarn, guarding the heart of Ulduar.
Then we have Auriaya, presumably given the similar task.
We have Archaedas, being the ultimate guard of Uldaman, protecting the Discs of Norgannon.
So, this leads to the logical assumption - Titans never leave anything important without a powerful watcher/Keeper.
Engine of Nalak'sha is one of definitely important things, right?
And of course, of course, you could say "well, you know, buddy, but.. hey, we have, like, Elegon, you know..?"
Of course we have! But isn't he a little not-Titanic? He's celestial and all, but his shape... Resembles the land, but wouldn't Titans rather have someone of their own kind? Like Ra-Den?
For me, he was appointed as a guardian of the Engine of Nalak'sha. Now, there are several speculations about the Mogu contribution in all of that.
- were they originally meant as a Titan artificial race, Earthen alike?
- did they just marched in, seized the power of the vaults, defeated and imprisoned Ra-Den, seeking his wisdom, and finally appointed one of the native beasts, enhancing it in the process to become Elegon?
No matter how they actually accomplished being in charge of the Engine, it looks like Ra-Den is its former watcher, both from model and the general "feel".
Now, why is he bleeding? Stones do not normally bleed, right?
Mogu are known for their savagery and lust for torturing (*ekhm* Krasarang *ekhm*), they just encountered someone otherwordly, Ra-den is a fascinating subject for them... Assuming they're either connected to the Old God itself (speculation), or just familiar with Curse of Flesh, having normal bodies before (another speculation), they COULD have a way to "play" with it, make him suffer in order to getting information. Sounds Mogu-ish to me.
Final thought, as for the last boss. Someone asked: if Lei Shi is such powerful and et cetera, why would the defeated, weakened, and presumably imprisoned Titanic watcher be a heroic-only boss? The truth is, we don't even know the type of the fight. Someone mentioned it could resemble Valithria..
Gathering his blood in a timely manner in order to him regaining his strength and retaking the vaults and the Engine? WHY NOT.
Possibilities are endless, they just don't differ from each other, as some of you thought. They all can be very well true and still connect to the others in some way.
Keep up the good work, guys, it's amazing!