It is possible to submit proposals that are not based on the ideas list, but I fear it'll be really hard to find a mentor :/
You would basically have to find a Flutter / Dart developer who has time and is willing to mentor the project.
If we had more mentors we would probably have even more ideas on the ideas page...
To be honest I don't think we'll have mentors for all the ideas outlined on the ideas page.
At the end of the day, we are limited by mentor availability and probably also slots allocated by GSoC.
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Regards Jonas Finnemann Jensen.