I'll admit, I have not been keeping up with the fantastic four comics lately, or ever, for that matter. But this thought occured to me when I was thinking about which one would die the other day...and it's likely ludicrous, but amusing non the less.
 
What if. They killed of the Human Torch, so that when/if Captain America meets the Fantastic Four in a Marvel Movie, that Chris Evans only has one part? Obviously if he is still alive when this happens then there will be some awkward recasting issues, but you get the idea.
 
As I said, likely ludicrous, but an amusing thought, no?

The film rights to the Fantastic Four are held by 20th Century Fox, not Marvel, so Captain America will not be encountering any of the Fantastic Four anytime soon.

Even if Marvel do get the rights soon, the F4 cast will probably be changed completely, so this isn't an issue.

Rest In Peace Johnny Storm :(  you will be missed even though you weren't as important as the others, you still are family and part of something. I wonder If they are going to replace him with anyone, I know Peter is supposed to join the Fantastic Four but that probably wont last very long.