Re: overweight passengers, if I ran an airline, there would be a seat with side panels to test your fit. The seat would have side pressure sensors. If you can sit in the boxed seat without touching either side, fine. If you can't do so without touching one side or the other, you'd have to pay more. We'd also charge for weight, something per-pound. It costs more in fuel to move each additional pound.
There would be a required training course for boarding and unboarding which all potential passengers would be required to pass. We'd also teach people how to put their carry-on in the bin in the correct orientation (with the widest side inserted depth-wise and the second widest dimension vertically such that 4 bags can fit into a normal bin instead of 3 or 2 + that one person's purse which should be under the seat). It'd be like a marching band, with timed choreography. We'd board using the front and rear doors. I bet I could reduce average boarding time to 20% of what it is today.
There would be no in-flight screens for entertainment, no drink or snack service, etc. There would be a double-walled [possibly plexiglass if no lighter alternatives exist] section for people traveling with children under about 12, with a bit of space reserved for anyone acting like they're under 12. This would contain the noise and smells. If this could not be made to work, simply restrict booking to passengers older than 12, then everyone else can appreciate baby-free flights.
The benefits of all of this would be reflected in ticket pricing and clearly-stated business practices + expectations. It would have limited appeal, but excellent customer service and I would expect a cult following of loyal frequent travelers who appreciate efficient travel.