Each plane model should have an effect on SQ and max stars on the route. This would add more variety, strategy, and flavor to airplane choice. It would be a good way to distinguish new aircraft and to carve out distinctive niches for budget and premium airlines.
P.S. I would also like to see more incentive to maintain fleet commonality with fewer free aircraft families and increasing costs of adding more.
I think I've seen these suggestions elsewhere - I make no claim to originality.