What about:
- Expanding nozzle
- SERNs and/or linear aerospikes
- Expansion deflection nozzle
- Plug nozzle (truncated aerospike)
The plug nozzle paper has exclusive access, but the first page has a lot of information
EDIT 9/9/2019: Now that SpaceX has confirmed the use of dual bells, those would be a great addition too
This post has been merged with this post.