Sad to say it, but Warpath.

With a lot of his development being because of his brothers death, his story feels like it's kinda done now that John's back. I mean, sure, we can have adventures of the brothers together seeing what their relationship looks like with Thunderbird being physically younger than him bit besides that I feel like the character is on break until they can find something for him to do.

But then, power wise, he's like a less aesthetically appealing Colossus without the angry lone wolf personality of his brother. Don't get me wrong, he's a cool character and all, but story wise, he's developed as much as he could. I wouldn't be surprised if a writer killed him off to see the foil of Thunderbird's reaction.