Hi Fotis,
An idea I had recently for K2 I think would really give it some extra kick: Installable 'data-sets'.
What I mean by that is allow for developers to create a specific setup (i.e. categories, custom fields, K2 templates, etc.) and then they can package them up and others can install them on their k2 installation.
For instance, a template developer could provide, alongside his template, an installable data-set that has pre-built fields, etc. as if it's a ready-made product (i.e. car directory, church membership roll, etc.). Lots of potential.