Originally Posted by KangConquers
Vulcan actually ticks all the same boxes as Quicksilver and Scarlet Witch. He's related to famous X-Men, and he's a mutant. However, many of his stories (including his crowning story, War of Kings) were told outside of the X-universe.
Sony probably has the rights to Hydro-Man just like they had the rights to Sandman (who also fought the F4.)
Beetle is probably a split rights case, though neither company has a huge incentive to use him right now.
Keep in mind he's written as the third summers brother and Mister Sinister has been hinting at his existence long before his first appearance. I'm not saying he doesn't meet that criteria just think if this were to be disputed in court that Fox would come out on top because Marvel already knew the terms arranged when they conceptualized the character, whereas QS and SW were conceived 30 years prior.
If they ever made a Thunderbolts movie Beetle could be a good choice protagonist. After all I believe his origin involved him stealing from Tony Stark and Marvel has traditionally liked to give their movies some sort of link to Iron Man.