This is a Thought Experiment.
There is no answer, AAA games with all the funds at their disposal and every madman/genius inbetween has failed to provide a more compelling business model than premium crrency/loot boxes, and by more compelling I mean a system in which generates a significant increase in income for game companies than the current premium currency/loot box system in wider general use. We are likely to continue to argue about grind vs reward until we are blue in the faces and someone else innovates for us. But, ever the optimist, I'll ask the question and continue to live in hope.
The only reason we have Astral Diamonds is because of the ability to purchase Zen with real cash, the reason we can't buy stuff with real cash is because 1) players would think twice if they see the actual cost of what they are purchasing 2) anything bought directly comes under consumer protection laws, if you buy currency, regardless of what you purchase with that currency, the digital currency is provided as is and exactly as described.
If we want a better system of earning ingame rewards we have a serious problem to contend with.
Removing Astral Diamonds and the Zen Market and replacing it with a better business model that performs better than the previous one plus the added income from loot box mechanics.
If we want to attack this problem seriously there are a few issues I can think of that need addressing:
1) What incentives drive a player in games which do not have in game premium currency, especially those that often replayed and are there examples of MMO's that have mimmicked said games and continue to maintain interest.
2) How do you monetise incentives that are not invasive/intrusive or provide an unfair advantage, unfair defined by distinct seperation of categories between those who invest time with those that invest real monies.
3) What incentives currently maintain player interest in continuing to play this game for an extended period or rather continue to pay for those that won't/can't and is there room for improving incentives.
1) Problem needs to be scaleable to any game that could use a F2P method.
2) No prophecy logic, so no arguments which can be ground down to say 'build it and they will come', if you have an answer, bring math, lots of math.
3) Make your points, clarify your points if you have to, do not argue your points.
4) Remember, no one thus far has found a solution.
Realistic outcome scenario:
Someone will come up with an idea that can enchance this games income AND increase player enjoyment.
Best case scenario:
Someone phones Keanu Reeves [The One] and he posts the answer.