I've been interested in the Hansa for some time (largely due to EU3, but I try not to think about that due to being a recovering gaming addict). So let's see if we can make a Hansawank.
The goal is to ensure that an entity directly descended from the medieval Hanseatic League survives into the present day. I have three ideas for how this might work:
1. Hanseatic Republic: The Hansa become increasingly centralized until they are a true merchant republic in northern Germany. Said republic manages to survive to the present (presumably turning into a modern republic along the way).
2. Hanseatic Colony: The Hansa launch their own expedition to the New World, establishing a colony. Said colony becomes an independent nation in the New World (or maybe joins a larger political entity).
3. Hansa Inc.: The Hansa somehow stay in the merchant shipping business, ultimately becoming a modern corporation.