MS SQL Server is required. It's a matter of both required features and differing SQL dialects.

How I understand Sql Compact should be sufficient than. On database level I see only tables, view and several stored procedures (which btw will be quite easy to replace by c# code)... 

Sqlite of course will be good to have too...

On my opinion simple migration of episerevr to any ORM that supports multiple databases will only benefit CMS.

Might not be exactly what you're looking for but you can store the content of pages in a different store, for example RavenDB - by creating a custom Content Provider. 

Thanks, good to know all possible options!