Personally, I'd leverage on PostgreSQL rather than MongoDB, because I think relational databases are still the best fit for the majority of modern web applications.

Unless, of course, you have a really simple schema so that you want to use the DB as a mere persistence layer. Or you know from the beginning that you'll have to scale really huge. But then, you'd need a highly customized framework anyway, which would somewhat defeat the purpose of writing a general purpose framework in the spirit of Django.

Also, PostgreSQL is allows to be extended from inside, thus working much in the spirit of Lisp. It can be extended with plain C code, but has also language support for pgSQL, Perl, Python and Tcl:

http://www.postgresql.org/docs/current/static/server-program...

Unfortunately, language support for Lisp languages is missing yet, but this might be only a matter of time (and would be really great).