Ideas of new features that would be very valuable to build into Piwik 4.0.0 or during 4.x development cycle. We've discussed these internally and this is just a list of exciting and most useful improvements.
Of course I like new features like "Compare report data for two different periods" very much :-)
Maybe one should spend same amount of work also on 3 other fields:
making existing functionality even easier to use/to understand for "average users" (not every one is a high power user):
have a look at the open issues with label "major" and "c: usability"
there are some really interesting ones ;-)
fixing bugs and messed up things in the field of data quality / data presentation