If it was warring triad, they would have just told us. It might be Nidhogg and Nidhogg EX though, that is 100% a possibility but I think they've strayed away from EX story stuff (though if that's all they have then that's what they'll do).. I just remember similar coyness when it came to the Gilgamesh and Ultros trials.
They were pretty open to EX versions of story stuff up through 2.4 and Shiva, and we did get an EX version of Thordan eventually, so I dunno. I lean against Hildebrand largely because you have to do the whole 2.x Hildi series to get there, and they've been pretty big about not locking the trials in HW behind lengthy prerequisites.
An EX Hildi fight would be amazing, though. Actually... add Battle in the Big Keep and Battle on the Big Bridge to things I would be fine with them rehashing