A new FOF type: Clip
It's basically an invisible solid FOF, except when you collide with it, as far as far as momentum, control, and playerstate is concerned, you haven't collided with anything.
i.e., if Tails is flying and lands on top of one of these, he'll still be flying, just not moving downwards.
Or if Knuckles glided into the side, he'd keep gliding, and upon moving out of the way, he'll retain previous momentum - no need to accelerate again.
Another thing that could be done is that setting NOCLIMB on the FOF linedef causes you to naturally move away from the center. In the previous two examples, Tails would start moving towards the edge of the FOF, and then would start falling, and Knuckles would also move towards the side. The linedef length could set how forcefully the movement is.
This could be used for more elegant and modern blocking of going to certain areas - if you don't want people to climb or stand on top of that tower you built as scenery, you could just cover it with this FOF, and people would naturally go around it, without it obstructing gameplay as much as a regular FOF.
And of course, NOCLIP makes you pass through these.