Resources on Political Rights of European Citizens
Whilst the main emphasis is on enforcement of rights which exist rather than creating new ones, it is generally accepted that allowing European citizens to vote and stand in local and European elections in their country of residence but not in the national ones or in referenda is problematic. (Commission recommendations)
DG Justice also publishes reports on the use or rather underuse of existing political rights and how voting could be made easier:
It is also useful to look at analyses of turnout and voting in the 2014 European Parliament elections and proposals for reform by the AFCO Committee: Working Document on the Reform of the Electoral Law of the European Union.