Hi Ansgar:
I was reading about the upcoming Percona conference and learned about some great new features coming in mySQL 8.0
You may wish to take a look at: http://mysqlserverteam.com/the-mysql-8-0-0-milestone-release-is-available/
The number of enhancements are voluminous and I anticipate they will make mySQL much better in many ways.
Together with the HeidiSQL community's calls for improved support for PostgreSQL and new support for other DBs such as SQLite and Firebird you are clearly at risk of being overextended.
Did you ever consider developing Heidi on Github or a similar service? Perhaps that would generate a bunch of development assistance.