If debridement and bone spur removal is done and ankle stiffness isn't resolved is ankle fusion next step? is it to slow/stop arthritic progression?

Talk to your surgeon: Only a surgeon that has evaluated your ankle and your imaging studies can answer this. Joint fusions are typically reserved for severe arthritis that has failed all other treatments.

Many options: The treatment options depend on your symptoms and the amount of pain. Ankle arthritis causes stiffness, pain, and sometimes instability. An ankle fusion is the removal of the joint with the intention of allowing it to become one solid bone. This may increase pain, but will increase stiffness. Other options include bracing or shoe modifications, ankle replacement, among many others.

Fusion....: Should always be a "last resort." there are now a number of different ankle joint replacement options as well. It works similar to a knee replacement - you would rather have motion there.