Oh my .. what a great project! I look at pushchairs and every single bloody one I think 'that is poor design' for at least a few features. Right...
Go look at the babyjogger city mini four wheeler. See that folding mechanism? That is brilliant _ you need something you can fold and unfold one handed. Lots say they can, but most can't. One handed means 'holding a baby in one arm.' It's got to fold down to a compact size and flat and be free standing when folded.
It's got to be light but sturdy - roughly 7-9kg seems to be the sweet spot. The frame needs to be rigid enough to have no wobble. Make sure it doesn't tip back if bags are hung on the handlebar. The hooks on the easywalker mosey handlebar are fantastic,
Single handlebar. Good steering. The handlebar should rise/fall/pivot to allow a short and a tall person to use it. Push it and stride - feet hit the back axle? You don't want that.
Reversible seat - big challenge to get that with the good fold. That's probably your biggest challenge tbh.
Decent sized basket that is accessible.
Decent sized hood with peekaboo window. Decent, easy to use rain hood.
Narrow enough to get down a bus aisle (54cm or less.)
Detachable, interchangeable wheels so you can have a true off roader or a city pram. Suspension has to be ok.
Another good bit of design to look at is the easywalker mosey. Almost, almost perfect. But.,, the fold isn't one handed. The suspension is poor. No window in the hood, the fold isn't compact enough (handle juts out) and the handlebar should slide further into the frame AND pivot.
Easy harness and buckle. No one wants to fanny around trying to buckle a toddler in. Easywalker mosey buckle is great.
Seat has to lie flat (not the bucket seats where their knees are in the air!) and sit bolt upright with a few positions in between. Seat has to be comfy, and fit all sizes. Detachable bumper bar is good.