The devs should use this as an opportunity to encourage more horizontal leveling (ranking up more than one class).

Give each class a class quest at rank 20 and rank 30 that grants one additional accessory cost point. This would give everyone a reason to rank up more than one class.

If you rank up every class to at least 20, you'll have 30 slot points, and if you rank each class up to at least rank 30, you'll have 48 slot points.