Sort:  

I don't think a fixed witness pay makes sense.

Part of witness pay (arguably the bulk of it) is for faithfully serving and securing the chain. If you have (hypothetically) $10 billion chain and witnesses being paid say $30K/year, that is a recipe for disaster not only in terms of scaling costs but also security. It may be fine when the value is only $100 million. But as the amounts of value at stake scales up, witnesses can easily be corrupted by outside influences worth more than their $30K/year position, but with significant negative costs for the success of the chain. You really want witnesses to be very well paid if the chain is worth a lot, ideally with more to lose from losing their witness position than to gain by acting against the interests of the chain/stakeholders.

I think a reasonable percentage per year makes sense, something likely in the 0.5% to 1% range, although the exact numbers may be up for debate in the future when we have more understanding of blockchains being used and critical to a rich, mature, economy.