I am not that familiar with debian processes, so here are some questions
about that:
1. Will the package be in unstable first? Or in experimental? Or
somewhere else?
It will go to unstable. If it sits there for 10 days without any serious
bug being found it will automatically be moved to testing (if all its
dependencies are there). Then, eventually (ETA: 2nd half next year, but
that is just my rough guess), testing is turned into a new Debian stable
release.
Any new versions go to unstable and subsequently, after 10 days, to
testing, until the freeze (which is expected for June 2012). At that
point, only bugfixes are allowed to enter testing, and need manual
approval.
So in summary: The last upload before June 2012 will likely end up in
the next Debian stable release.
2. I understand, that in "stable" packages are not updated to newer
versions (except for security and stability fixes). As our game is still
in development, there will be more or less frequent updates. Are there
branches that are more suitable for this situation?
Right, the version that ends up in stable probably won’t see any
updates, so it should either be already usable on its own, e.g. with the
included levels, or not part of a stable release.
There is also a backports repository with the policy that packages from
testing can be built against the stable release. So newer versions could
be uploaded there and hence be available for users of Debian stable.
If this is not possible for some reason, then you (or we) can still
build newer versions of nikki for Debian stable and put them in an
unofficial repository on code.joyride.de or somewhere else.