You could do it with a whitelist. If there is a fork, give disproportionate weight to blocks mined by a whitelisted participant when doing the longest-chain calculation. Ideally you should include the proof of being on the whitelist in the block itself, but if that's not possible for some reason you could always send the information off-chain.
That's centralization, which is the opposite of what's intended and has its own risks. Most blocks are mined by pools, so you'll have to whitelist them, and while you might trust the pool operators now, will you forever? You'll be making the cost of an attack significantly cheaper for them (or someone who steals their magic key, or tricks you into adding them to the blessed list).
I agree that it is not ideal. But addressing some of the specific point brought up:
1. a) The list doesn't need to be hardcoded, it could be a configuration. b) So trust doesn't need to be permanent. c) It could be decentralized in the sense of allowing different people to have configs 2. Miners not on the list can still participate just with lower weight in the case of a fork. And they still get full reward.
1. A cryptocurrency requires consensus, so no, you can't have different configs for determining the validity of a chain. Making it a config variable only makes it faster to close the barn door after the horse has bolted.
2. Has no bearing on any point I made.
What will likely happen is a PoS BFT layer on top of PoW, although there are other options being considered:
As long as people eventually reach the same conclusion about which chain is the legit one it's fine that they use different reasoning to arrive at that conclusion.
If they fail to ever converge there is probably such a large disagreement in the community that a fork is for the best anyway.
> As long as people eventually reach the same conclusion about which chain is the legit one it's fine
What? No, it very much it isn't. Consensus needs to be ongoing, within a handful of blocks (Monero locks transfers for 10 blocks for this reason, called "confirmations").
Firstly, I think you underestimate how quickly good faith actors with slightly different configs would come to agree. A handful of blocks should be enough. Secondly, if reorgs start becoming a problem, exchanges and merchants could monitor for a situation with two competing chain and temporarily suspend processing. There is still the possibility that some one will suddenly reveal a long chain they had kept secret, but anyone doing such a thing is very suspicious.
