Suggestions for improvement

[highend]

The better approach is to handle lower and uppercase A-Z as different jump marks. If you have a large screen or a small font there can be many matches (more than 26). With a-zA-Z you have 52 different positions you can directly jump to. The Easy Motion plugins that I know of (for ST / vim) do it the same way.

Btw, what is supposed to happen when there are more possible jump locations than (atm) 26 / 52 (for a-zA-Z)?

A good way to handle it is: If that's the case, the spacebar shifts to the next results (so the current positions are hidden and the next matches will be visible).

[rr-]

Btw, what is supposed to happen when there are more possible jump locations than (atm) 26 / 52 (for a-zA-Z)?

Vim shows you combinations of two letters, like bh. Hitting these consecutively will navigate to the target.
This rule applies recursively, i.e. if all two-letter combinations are used, three-letter combinations appear (try testing vim-easymotion with a long file that contains only letter a).

I think it's much simpler than navigating with space between result sets, since you know what to press straight away.

[JamesVannordstrand]

I know it is a little late but I have added a pull request for double key combinations. Hopefully it will be merged in and released.