I feel like there's other options, it needs to be developed by very, very smart minds though. Implementing changes to how our government works, in terms of either additional checks/balances, limits to certain aspects of certain groups, or even fundamental changes in requirements such as always requiring at minimum a 3 party system could potentially limit the capability of something like this happening again. And by 'something like this' I'm more generally talking... not just the pres firing the DAG to avoid his own investigation.
I do feel like that needs to be an immediate and clear statement by congress though. A president flatly cannot obstruct justice on investigations toward him, or any that may have colluded to him. If someone's under investigation, the pres should *not* be able to halt that investigation. I would include pardons in that.