Mr. Fantastic is not a vanguard actually makes a lot of sense from a game-play perspective.

People expect Mr. Fantastic to be able to stretch his body or hands to shield his teammates. However, the problem is, since he cannot turn his body invisible, his body is going to obstruct the players' views for both sides. I think this is why he ended up as a DPS.

This is why Magneto uses a transparent force-field instead of a solid metallic shield.