> small size
These are conflicting requirements. The minimum size for 6 doors is probably about the size of a long-wheelbase Transit van, only a little smaller than a normal bus.
I'd love to see someone try the commuter multi-drop-mini-bus system; it's implemented informally in various ways already that don't have multinational branding. Won't help in really dense cities like London where traffic is at walking pace, but potentially useful in other places.
Depends on the mini-bus layout. 7-seater VW Touran (4527 mm long) has 5 doors. So maybe it possible to build 5 m long van with 6 doors, with 2 on the back.
> Won't help in really dense cities like London where traffic is at walking pace
Dedicated lanes and some regulations will be needed. For example mini-bus shouldn't be allowed to enter crowded routes unless it has 3 passengers. So two people would have to wait 5 min for the third passenger and so on. Or it can be used to transport people from suburbs to subway stations in cities like London.