The way to make if fair between current and future participants is by not making changes the the fundamental economic structure at all. That creates an equivalence between people who are involved now and people who are involved in the future in that neither get to vote. This in turn avoids future participants feeling disadvantaged (and thus discouraged from participating) because they weren't around when important economic decisions are made. It also avoids people not wanting to invest as small participants because they risk getting screwed over by larger participants voting their own interests.

As you point out, you can't make it fair by allowing both current and future participants to vote, so you make it fair by allowing neither to vote.

The sensible way to address not liking something about the fundamental economic design a coin is to create a new coin. Use the claim method to preserve the existing distribution if you think the existing distribution is a good starting point. That has the same economic effect as forking the chain except that you avoid having transactions that confirm on either or both chains, leading to chaos.

This, BTW, includes the idea of whether changes can be made by voting. If you want a coin where the design states that coin holders vote by balance on future changes then create one, but put that in the design from the start, so again everyone is entering on equal terms.