The optimal design to replace current commuter buses would be something like the following. Multitudes of small, low floored 10 seat (incl. driver) minibuses, the low floor facilitated by a front motor and front wheel drive, that would require at most marginal step up from a standard curb and have autoramp capacity for the passenger door.
Buses would allow 2 metre headroom in a central passage, being emergency standing room and have a laterally arched roof to minimise sway mass, (the useless portions near the top sides of squared off buses that causes them to sway side to side whenever they turn, increasing subliminal discomfort,). All seats would be single window seats (urban people usually travel alone, simply look at any half-full bus, or the majority of cars, to confirm this), the only exception to this being three seats across the back, and with space opposite the door for a wheelchair or pram. This bus would be narrower than existing "people movers" and able to negotiate overtaking each other on narrower roads they share with recumbent cycles and service/delivery vehicles.
The seats would be lengthwise spaced by a full metre and supported from the wall, not floor, to maximise convenience in baggage setdown and pickup from the space under the seat, and facilitate proper cleaning. This would require rigid bracing from the wall frame (which would increase the strength of seat attachment and reduce their uprooting in collision) with some spring or gas mounting under the seat-pad, the expensive gas mount would allow height adjustment, but isn't crucial.
The two front window seats would be preferentially reserved for the infirm and the back 3 seats for parents with small children, minimizing parental stresses by boxing children into many windowed corners and not affording them the opportunity to get behind the parent's back.
Tickets should be valid over the whole web for varying periods of time, and purchasable from any bus driver, but their validation would be via an automatic machine.
Of course beneficial technologies, such as fuel cell or hybrid electric/gas engines should be incorporated in these vehicles.