Right now the main problem is parallel apply: we need to apply changes concurrently to avoid unintended dependencies causing deadlocks and provide reasonable performance.
How do you intend to approach that?
You should be able to do parallel apply between nodes trivially, i.e. node A applies changes in parallel from nodes B C and D.
Doing parallel apply of multiple changes from node A to node B is much harder. I wrote about parallel logical apply separately so I won't repeat it here; search the archives for the notes if interested.
We also need 2PC support but this code was sent to you by Stas, so I hope that sometime it will be included in PostgreSQL core and pglogical plugin.
I never got a response to my suggestion that testing of upstream DDL is needed for that. I want to see more on how you plan to handle DDL on the upstream side that changes the table structure and acquires strong locks. Especially when it's combined with row changes in the same prepared xacts.
I'd really like separate patches for the core changes and the pglogical support for them, too.
There are also some minor technical issues which lead us to making few changes in pglogical code but we tried to do our best to keep original versions unchanged, so we can switch to public version in future.