Sound like we are sharing the same goals for a library :-)
What about this plan?
* I contact OSGeo for asking permission to re-license Geotk "core"
under the Apache license (the "pending" part may need more
negotiation with OSGeo; it would be a next step).
* We re-license Geotk in their current repository
(http://www.geotoolkit.org).
* Apache SIS can consider the Geotk code and migrate to SIS only the
parts that SIS selects, on a case-by-case basis. This will probably
need a discussion on the overall architecture first.
* Code migrated to SIS will be replaced by a dependency to SIS in the
Geotk code base. This allow a migration at whatever speed peoples
want, without any pressure.
We will probably have duplication between some Geotk classes and
existing SIS classes; I guess we will need to resolve them on a
case-by-case basis.
We may also need a discussion about what should be public committed API,
and what should be internal mechanic available to the users "at their
own risk", and what should be the package names for each of those parts.
Martin