>How would you design a feet forward
I'd just copy pic related but with gears. In my eyes it's already the perfect design (center steer underneath your seat base so you can once more keep balance with your hips like on a normal bike, simplified drivetrain and normal chain length thanks to FWD, both wheels with dampened swing arms, good seating height, under seat steering, elegant design), it's just lacking more gears.
>that has the possibility of becoming mainstream?
Impossible, normies will never be willing to trade the upsides and downsides of normal bikes for those of recumbents, or else they would've already done so.