Famitsu mentions that Square Enix’s 1st Production Department is handling Final Fantasy X HD. The development team is starting to work on it, which is why there haven’t been many announcements.
It’s interesting to note that Square Enix is handling this internally since remasters are often handled by third party developers. Sanzaru Games took care of The Sly Collection, for example. Bluepoint Games (unofficial masters of the remaster) did God of War Collection, Metal Gear Solid HD Collection, and Ico & the Shadow of the Colossus Collection. Square Enix usually lets third party developers work on ports too. Tose handled the Final Fantasy Advance series and Bullets Ltd polished Final Fantasy IV: The Complete Collection.