Suggestions for improvement
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).
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.
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.