You cannot do a complex or interesting layout in Leanpub via our normal ebook generation of PDF, EPUB and MOBI files.
However, you can do as complex or as interesting a layout as you want if you use our InDesign export feature.
The thinking here is as follows:
Leanpub is for publishing in-progress and completed ebooks. While the book is in-progress, you should be writing, not formatting, and Leanpub should save you from having to do any formatting by providing a simple, minimalist layout that is good enough. Furthermore, complex and interesting layouts often break on mobile.
When a book is done, we have a print-ready PDF export feature to produce a print-ready version of our minimalist layout. For many books this export is perfectly fine, and is sellable as-is on Lulu or CreateSpace.
However, it doesn't look custom designed -- not even close. For books with a design budget or design talent, nothing we do algorithmically will be 100% satisfactory. So, we provide the InDesign export, wherein we produce ICML files.
This way, we don't have to do any layout support at all in Leanpub, since (a) it would detract while the book was in-progress and lead to procrastination and (b) anything we did would not measure up to what Adobe enables talented designers to do anyway...