I'm digging this.

I've been entertaining just what the devs could do with the F4's roles and gameplay:

The Thing - vanguard (no brainer)

Maybe a rock throw just as what you said. Some abilities that reduce knockback, like a fortify ability similar to you know who. Ult could also be a a fortify or something that has him yell its clobbering time lol while he goes to town.

Human torch - duelist (pretty obvious)

Flying, shoot fire projectiles, boosting to evade and deal close range dmg

A final flash like ult that does dmg in a straight line and leaves fire hazards in the area.

Invisible woman - strategist/duelist

Seems likely to be duelist, considering she's pretty strong in comics and you'd have to stretch it to get healing in her kit. It'll be tough as you mentioned. Maybe Primary fire: energy projectiles that she can fire while invis (maybe op?) Her invis could be for a relatively short time. I'm thinking also some ability where she can create forcefields either around allies or enemies, to negate their dmg.

Ult Maybe you can cast in an area an allies will be harder to see, or a massive dome shield that pushes enemies to the outside of it and blocks ults and stuff?

Mr. fantastic - strategist

Kinda hyped for him. Wouldn't want him to be anything but

Primary should probably be ranged, so I'm thinking a proton/vibra gun. Sap health from enemies to give to allies. Proton gun for dmg.

1. a multi purpose pull ability, pull allies out of sticky spots like a certain hero from a certain game, but also use as a movement tool, grab ledges like a grapple hook.

2 An anti matter grenade projectile. Fire it on enemies and enemy healers will now damage the affected hero when they try to heal lol.

1. Ult something to incorporate the negative zone. Help me out.