There are some planned features of Necessities that would need to tie into the API, and I would rather not have one module be a requirement of another module.
Plus, due to the way that I am structuring the code, disabling the economy parameter in the coreconfig.conf will prevent any economy code from running.
That introduces the need for two plugins instead of one. I understand if you would want people to program against the economy API in the core instead of the implementation so that the implementation can be interchangeable, but is that really necessary? In the end it’s up to you and what you want to get out of this 
I just want to make it easier for server owners. If I were to make the Economy API its own plugin, then other necessities modules would require the API and it would end up being another core.
And what if youmaintain it seperately and then fatjar it in? 
What exactly does that entail? How would one go about doing that? What does that mean for the coding?
It means that you handle the Economy API as a seperate project entirely (of course yor other plugins can depend on it, but not the other way round) and when it comes to distributing jars you just put it into the same jar as the component of your plugin that requires it. I don’t know what you use for building, but in gradle i know that there are several ways of creating a fatjar, there is even a plugin just for it.
Oh, by the way an Economy Service is already being implemented within the Sponge API. I think you should do some more research.
Is there a rough idea when this may be?
Last I heard, KHobbits was having difficulties putting a team together that would have the time to do the porting…
Sadly, it seems that Essentials is done. We would possibly work with one of the main general command plugins on Sponge however.
That’s gutting… I loved GM as well but now have to resort to PEX which in my opinion is not easy to understand…
There’s a couple other permission plugins available you might find more fitting