I think we need to focus on things that _can't_ be done first, rather than things that require porting, e.g. until we had savepoints, you couldn't migrate an application that needed it. It wasn't a question of porting --- there was just no way to port it.
Those _missing_ pieces should be a priority.
Nested/autonomous transactions? Do they occur often in PL/SQL code?
There is relative well working workaround - ora2pg is able to translate it to dblink usage.
Sure - native solution can be better - usage pg_background is step forward.
