Hi Matthieu,
Thanks for your email, I can understand your frustrations about wanting to see MongoEngine progress.
First and foremost MongoEngine is an open source endeavour and it relies on people giving up their free time to maintain and work on the project. There are many competing pressures here as for many users the Pull Requests might not be blockers and they are happily using the project or working round issues, so there is no equal sense of urgency.
A little history about MongoEngine might help with the context. I started maintaining MongoEngine because the original author Harry Marr had moved to a new job and a new language. No one was maintaining the library actively and effectively the project was dying - its the nature of open source. I needed a Django like ORM for MongoDB - I didn't need Django just an easy way to define schemas in a flexible manner and a way to generate forms from those schemas, so I took up developing MongoEngine and became the maintainer. From that work I ended up joining MongoDB (10gen as it was then), I worked on the python driver, gave talks at conferences and continued to maintain MongoEngine. Since then I have changed role and language, I now work on the JVM and due to personal reasons I have no longer have any free time to work on MongoEngine.
Realising history was repeating I asked myself how can I stop MongoEngine from falling to the same fate as before? Perhaps it should befall the same fate as it needs a champion - maintaining a large open source project can be a lot of work. However, I searched for possible maintainers and repeatedly solicited help on the project, for a long time there was little or no answer to the call, people would come and go as their needs changed with MongoEngine. Last summer I finally started to see some success and now there is a core that actively work on MongoEngine and are working with the issues and pull requests.
I would like to thank those collaborators (in no order: Serge Matveenko, Yohan Graterol, Omer Katz, Wilson Júnior, Bernardo Heynemann, David Bordeynik, Alon Horev) for spending their own time in helping the project and helping move it forward. They are spread across the globe in different timezones and have multiple different first languages but all share a wish use and improve MongoEngine. There may not be a single leading maintainer but I see that as a good thing - I've seen different people using MongoEngine in very different ways, so having a spread of experience will benefit the project.
As for how the project is managed I can no longer help, I'm not the maintainer and I don't have the time but what I do know is the best way to drive it forward is from the inside, you are obviously passionate about it and I'd hope you could use that passion to help improve MongoEngine in the future.
Ross