The Moto X marked the spot for the the company's Google-centric rebrand earlier this year, and it looks like the naming convention may stick around for future models. The US Patent and Trademark Office is showing a new trademark filing from Motorola: the "MOTO G." This doesn't indicate that a new phone is coming, but it does mean that Motorola is interested in using that particular name for a future product.
(The USPTO doesn't allow direct links to trademark pages. If you want to check our work head to this page, search for "Moto G," and click the first result.)
The filing points to use for "Mobile phones, smartphones and accessories therefor, namely battery chargers, adapters and removable covers," and it was submitted to the USPTO on October 15th. The only other post-Moto X product that we're aware of is the Moto DVX, presumed to be a cheaper variant of the X for the international market. The Moto G could be literally anything, or indeed nothing - companies often trademark generic terms to cover their bases and sit on the filing for years.
Of course, it's hard not to do just a little bit of speculation. Don't read the rest of the paragraph unless you understand that we have absolutely no reliable information beyond the trademark filing above. Still here? Alright then. Motorola has mentioned that they plan to offer a version of the Moto X on the Google Play Store at some point, presumably with AOSP software and rapid, Nexus-style updates a la the Google Play Editions of the Galaxy S4 and HTC One. "Moto G" (as in Google) would be a nifty name for that device, although it would also be the first time that a GPE device received a completely new moniker. It could also be a more "pure" Google phone in some other form.
Or it could be a completely different phone altogether, with only an abbreviated name connecting it to the Moto X. Hell, it could be a battery charger or a line of cases or an Android-powered toaster for all we know. We'll be keeping an eye out for it, whatever it turns out to be.
Source: US Patent & Trademark Office - thanks, anon!