Work on packages
Help fixing the system
You can help triaging and fixing bugs.
You can participate to Bug Squashing Parties.
You can join the QA team.
help maintaining packages
At this stage, you are surely part of a maintenance team.
The next step is to become a Debian Maintainer.
And then you'll have fullfilled your destiny when you are a Debian Developer.
But like always the end is only a new beginning.