Airlines should take a page out of Norwegian
Low fare = underseat carry on
Low fare plus = underseat+overheat+23kg checked. If overhead already full when you board it gets gate checked
Flex= underseat+ guaranteed overhead+ 2x23kg checked. Boarding before Lowfare plus
Easyjet do something similar, they charge for larger carry-ons (wheelaboards and larger backpacks). In fact they yield manage it so that when demand for overheads are high (and allocated space is filling up) they charge more. Often on business routes/timings can be cheaper to check a bag than carry on whereas for leisure routes it is more likely to be the other way around.
Not sure I'm a fan of the yield management aspect, but it does make a lot of sense to charge for larger carry-ons (or offer it as a status perk) and also seems fairer to charge a similar price to bring 15kg on board vs checking it in (yes there is the handling cost, but there are also costs associated with taking 5 mins longer to board because the o/h bins are jammed full).