I had a thought that the Sentinel mutant detection tech could be adapted from the stolen Cerebro 2 tech salvaged from Alkali Lake facility but dunno how plausible it is that anything salvageable survived.
The only explanation for Juggs superstrength and durability I can come up with is that he's been genetically enhanced. Cyttorak = good for comics bad for movies (at least in keeping with the syle of X1 and X2). Cain's never been the most social of people so he could be convicted felon that the government 'volunteered' for experimentation.
I like Juggs as a solo entity too, but I needed an explanation of how he'd get his psi immune helmet. Therefore a gift from Magneto, but anything from Magneto comes at a price. So I'd go with a compromise Juggs wreaks havoc alone first half, until he needs the psi-immune helmet at which point he's indebted to Magneto.
I'll say this scene happens maybe at the halfway point of the movie.Magneto hints to the Brotherhood that someone has been playing with his mind and it's not Charles.Magneto is walking through his lair and the next thing he realizes,is that he's back in Germany as a child.He sees his parents being pushed towards a huge building (a gas chamber).
His Father starts fighting with a guard and yelling for Eric.A guard then hits his Father in the back of the head with a gun.Magneto (now looking like a child),runs towards his father screaming Papa in German.
Magneto then realizes he's back in his lair.
Pyro:What's wrong?
Magneto:Be prepard children (adressing the whole brotherhood).
We have an unwanted visitor.
At this time Magneto puts on his helmet.Suddenly the cave seems to shake and all the Brotherhood,but Magneto,falls to the ground grabbing their heads in pain.Jean bust a hole through the cave and is consumed in fire.
Magneto:Just as the Phoenix rises from the ashes so does the X mens precious Jean Grey.What do you want?
Jean:Answers Eric.I know the Proffessor did something to my mind.What was it.
Magneto:Why don't you just read his mind and find out!
Magneto then creates huge pieces of metal and wraps them around the Phoenix.After a brief second,Jean bust out of the metal casing.
Jean:He's got shields within his mind.I can tear them down,but I would just rather get the answers from you.
Magneto:And what makes you think you can get through my shields?(points to the helmet)
Jean:Because they're weaker.......
Magneto's eyes become very big and his helmet shatter off his head.He then falls to the ground and pain as Jean probes his mind.
However,the liquid serum that controlled mutants was too powerful for Cain and Stryker pretty much abandoned the Juggernaut and left him under strict security at Alcatraz.
What if Juggernaut was apart of Strykers Weapon X project?Remember how Stryker was controlling Magneto and getting information?Let's say Magneto told him about his Helmets telepathy shield and Stryker used the same technology to harness a mask for his test subject the Juggernaut?
I think he needs to be one of the main reasons for a sentinal launch (along with the public finding out a mutant has been posing as a senator).I'm thinking Costume wise,Ultimate Juggernaut had a cool outfit.Maybe change to eye slots though.In Ultimate X men they where just one red slot (like Cyclops).In the movie we should see his eyes.