Hi,
Please ask all Marble related questions on the Marble mailing list (
marble-devel@kde.org) since this is the best place to get qualified answers. Otherwise we might just miss your question. :-)
Currently there are no plans to integrate Mapnik with Marble.
We do however have plans to render .osm as vector graphics natively using Marble. Depending on development-involvement that might take a while however.
Best Regards,
Torsten