Maybe that is already on the roadmap - but for the case it isn't, here is the wish:

Let's configure epub display to show the book in one, two or three columns.
Like this, the lines are not so long, and it's easier to read.

And maybe we could put better text/paragraph flow on the roadmap: most epub apps/readers I tried do line breaks even directly after a heading or do even break the heading onto multiple parts - whereas in correct page layout, the heading (not all, but most) is taken into account to not break them or at least make the break before.