What would happen if the miner reward was proportional to the difficulty?
As more and more people mine, the reward would be bigger and bigger, allowing for a more fair distribution of coins for late comers, IMO.
I think a good formula for miner reward would be reward=0.01√difficulty (square root to discourage inflation), which would give 5569 bitcoins right now, or 24 million bitcoins per year at the current difficulty.
Of course the early adopters would not like such a system, but they are the minority IMO. As long as the majority benefits from such a system, I think we can do it.
Well, your numbers show the reason why it wouldn't work; you say there will be 24 million mined each year, where Bitcoin has a hard cap of 21m Bitcoins that exist in total.
Also late-comers just have to deal with it; it's like everything. You find it early, take a risk, get rewarded sometimes and lose out others.
Find something new and get involved with it, then get rewarded. It's not supposed to be fair for everyone. First person to a new cache of resources gets rewarded appropriately.