While i was implemented k2 for my website, i improve (i think !) some functions and files from the core !
(look the post here getk2.org/community/New-to-K2-Ask-here-first/184245-I-dont-understand-the-itemRelated-by-tags-list)

I'm unable yet to write a plugin (it has no sense anyway) but i would like to share these improvements ...
- itemRelated by tags list with count and relevant (itemlist.php)
- harmonization of displays (category.php, item.php, category_item.php)
- bug (i think) in language files fr-FR_comk2.ini or gb-GB for the date template (for example %d %B %Y instead of 'd F Y' and some typing rules not right

So ?