Thanks for the responses, please keep them coming!

I thought of a couple of more:

What I would pay for: Ability for those who are non-Elite to pre-assign preferred seats (aisle, bulkhead, exit row, etc). Elites would get access to preferred seats for free. Their closest American partner, NWA, already does this.

What I would be willing to forego: Free alcoholic drinks in Economy. Heck, Delta (which is a fellow SkyTeam member), American etc have already done away with this, though KLM and NWA still do not charge for this.

While free drinks is nice, I do not think I would choose my economy class carrier on this basis... other things like fare, transit time, etc would be much more important to me.

Edited to add: Perhaps a happy compromise would be the first drink service would include a free alcoholic drink, but all subsequent alcoholic beverages would have to be paid for.