Wasn't Red Hulk one of the Defenders at one point?
He had a team called the
"Offenders" consisting of
Red Hulk, Tiger Shark, Terrax and Baron Mordo ... basically a villains version of the main 4 Defenders...
...Are you thinking of the 2011 Defenders team which consisted of
Doctor Strange, Silver Surfer, Namor, Red She-Hulk and Iron Fist?
It's not about it being a new element, but Rulk offers a unique deformation to the Hulk mythos that Venom and Winter Soldier don't offer. They expand the Spider-Man and Captain America concepts in new directions, not just add a powerset to an old foe.
No they don't
"...just add a powerset to an old foe"...
...Venom
"adds an old powerset to a new foe"
...and Winter Soldier add's the "foe" element to a former ally and beefs up his powerset.
It's not actually that different.
Also ... Iron Patriot = old foe improved with new powerset
...Venom (MacGargan) = old foe improved with new powerset
... Mach V (Beetle)
etc
When you said Rulk becomes an Avenger in the comics, it had the same effect on me as if you said Venom (Brock) started dating Mary Jane, or that story where Osborn got Gwen Stacy pregnant. It had the same effect as if you had said Winter Soldier become director of SHIELD.
It wasn't me that mentioned Rulk becoming an Avenger, but I have to say I think you're being VERY selective here...
What about Flash Thompson becoming Venom?
Tony Stark becoming Director Of Shield?
Colossus getting Juggernaught's powers?
etc
It seems like to support your argument
(which is one of personal taste) you're pulling examples out of the air that don't stand up to scrutiny, particularly when set against a broader look at the Marvel Universe.
Surely the MCU should be for EVERYONE? .... that means not pigeonholing it for the benefit of fans who only want to see the bits they have a personal preference for and nothing more ..... that's a restrictive approach.
There's something almost... incestuous about the Hulk family....
Absolutely,
(see "Old Man Logan"
)
Red Hulk exemplifies that, especially when he takes Hulk's arch nemesis as his alter ego and Hulk's role in the Avengers...
"he takes Hulk's arch nemesis as his alter ego "? .... nah I'm not even going to entertain the wording of that.
...It shows that it's all just looping back on itself, not an expansion like Venom or Winter Soldier. Red Hulk doesn't temporarily take over for Hulk... he's a Hulk annex of sorts. Hulk is still around being Hulk, but now Red Hulk is also Hulk, just red. Superman Red and Superman Blue come to mind.
The best things to ever happen to the Venom symbiote story-wise wer Mac Gargan and later Flash Thompson. That's an expansion, prior to that Venom's story could have been told with Peter as a "What-If" where he keeps the symbiote.
Rulk does more than that. There's more difference between his powerset and Hulk's than there is between Spidey's and Venom's

I don't know how you can call Venom more of an expansion on Spider-Man than Rulk is on Hulk?
As for the Winter Soldier .... he BECOMES the new Captain America.
Are you one of those people who READS the classics but READS ABOUT the new stories and decided not to bother before you've read em?
It's a little hard to define, but the feeling one can get from Red Hulk is that he is poorly conceived and executed. He could be like a Venom or Winter Soldier, but instead he's like a Jackpot (remember Mary Jane's superhero alter ego?) or a US Agent that the artists just can't get enough of.
I beg to differ,
I think if that's the impression you get of Red Hulk you must have read from his early appearances
(which played the "red-herring" card too often) up to his diluted presence as an Avengers member.
Check out
"Circle Of Four" and
"Marvel Now:Thunderbolts", that's Red Hulk in his element...
...But we aren't going to get that in the MCU without bringing him in as part of Hulk's story first.
I say get it over and done with and introduce She-Hulk and Samson in one film and Rulk, She-Rulk and A-Bomb in the next, then let em all toddle off around the MCU and do their own thing.
EDIT: I think I would do Red Hulk still, but I suspect I'd make it someone other than Ross.
That would remove the very essence of what the Red Hulk is.
To fight the thing he hates, he was willing to become the thing he hates.
Hulk is the
"monster" inside Banner...
...but in a way Rulk is the
"man" inside General Ross.
That's not going to have the same effect if you make Rulk the alter-ego of someone like Glenn Talbot
(who would be the next obvious choice) 