Threema puts more focus on business customers than in the past (as it's more sustainable than one-time payments, there are no external investors). But the business app is still fundamentally the same as the "private" app, just with management/configuration features added. Private users are not second class citizens.
Regarding open source, the app isn't unfortunately. But the crypto (based on NaCl by djb) is documented[1], has been audited[2], has been reverse engineered twice[3][4] and there is an alternative client implementation[5].
[1] https://threema.ch/press-files/2_documentation/cryptography_... [2] https://threema.ch/press-files/2_documentation/external_audi... [3] http://blog.jan-ahrens.eu/files/threema-protocol-analysis.pd... [4] https://media.ccc.de/v/33c3-8062-a_look_into_the_mobile_mess... [5] https://openmittsu.de/
(Disclaimer: Threema dev)
Providing an Open Source client seems no goal anymore.
What all the other messengers should copy is their poll feature [0]. It can basically act as an integrated Doodle.
[0] https://threema.ch/en/blog/posts/threema-poll-feature