Perhaps the question is: what do you miss in Nickel? What kind of experience or feature would you like that you don't currently have? What do you think could be improved? It might be that some of this is already in KOReader or something else.
Other things I missed (more or less strongly), and found in KOReader:
* True page (i.e. screen) numbers, with reliable (at least with my books) pages-to-finish and pages-to-next-chapter counters and/or ticks in progress bar. (By the way, see for example
this book for usable, in my opinion, PDF with the kind of progress bar I wanted.)
* Permanent clock while reading (optional).
* Style tweaking control without patching, survives upgrades.
* Reliable "back to previous position" action (I was tired of having to set a bookmark before viewing a footnote, in case the previous position got lost and there was no "back" link, which happened very often).
* More reliable annotation/highlight support, that doesn't disappear, lock the reader or refuse to highlight the fragment I want.