General developer forum
Surely 'constructivist' is a whole philosophy of learning and one which Moodle as a while is built around. It's not as simple as 'build a constructivist module'
What do you want the students to actually do? What do you want them to do that is not already catered for in Moodle... ?
Constructivist learning isnt about the tools per se, but about how they are used - some (like Workshop already mentioned, or forums, or chat) make it easier, but even lightbox galleries which allow comments, glossaries and databases that already allow students to create things together facilitate constructivist learning.
So, decide what you want the students to DO, then see if that feature already exists - it maybe that your new module would be a reworking or redevelopment of an existing feature, doing it better, rather than a totally new concept.
I suspect then that this is a subject to take back to your tutor:
Show him/her the research you have done into what 'constructivist' means, demonstrate your understanding of how that is already addressed with existing plugins (including existing core features) and ask for advice on what exactly the requirements for your work are - I believe though that you really need to be speaking to your tutor about this, we could possibly make suggestions (such as Marcus' above), but it sounds like you need to work with your tutor's understanding of what the expectations actually are.
Is this a student project or something else? What are your strengths in programming?
There are quite a few starter templates available on github for different kinds of project.
- Fix marginalia for Moodle 3.4. http://webmarginalia.net/
- Take an existing module and enhance it to improve collaboiration
- Inline editing for forums https://tracker.moodle.org/browse/MDL-33692 This will immediatley raise the level of forum replies since you can see what you are actually replying to at a thread level.
- Improve collaborative file sharing. eg List all attached files in a forum. https://tracker.moodle.org/browse/MDL-27062
Just three ideas!!