Ultima the High Seraph.

[Cleretic]

There's a reason I said "naturally-occuring primal." The deeper we look into the lore about Primals and aether, the more apparent it becomes that more than a few things are created as part of the same process of collective faith, something supported by dev statements. Auspices are probably the most substantial example of a "naturally occuring primal" where the collective faith of the far east is focused on animals because of the local religious beliefs and mythologies, but there's plenty of others, with Susanoo being the most blatant case where we actually SEE an example of a Primal-like being being created due to collective faith reaching a certain threshold, but plenty of others like Tural vidraal, tsukimogami, and even the archbishop of Ishgard all being examples of beings that were previously normal becoming abnormally powerful (the Archbishop was explicitely able to temper the Heaven's Ward even before he got the eye out for transformation.) due to the collective faith of a region. They're not all considered Primals due to particular details differing, but the underlying principles which result in their creation are all consistent.

Well you're not describing a helpful distinction, then, because you're mixing actual primals (Susano and King Thordan) with things that aren't primals (the Auspices and Tural Vidraal). And in fact the Auspices aren't really comparable, because it seemed to be that they actually don't get power from worship, they just get power from living for so damn long that they collate enough aether to become comparable in power anyway. Some of them are worshipped, but that seemed to come afterwards; you see a big flaming bird and build a religion around it, rather than building a religion that involves a flaming bird that then gets summoned. It's entirely possible that worship is the source of the difference in behavior between Auspices and Tural Vidraal who definitely aren't worshipped, but at that point we're dipping into vibes-based speculation.

Now, the actual comparison point that's valid to bring up is the Twelve. Their process of deification was also explicitly compared to primals, but they also aren't primals (again, Triple Triad cards don't mark them as such, but also the story makes it pretty clear itself); they were extant figures of great power, that then got shaped by a lot of collective belief over a lot of time. That does sound roughly like Ultima, especially if we add in the evidence that Ultima was on some level of 'around' since before the Sundering as evidenced by Pandaemonium. We don't know when Ivalice was, but it's safe to say that thousands of years passed, which would give Ultima a similar series of conditions to the Twelve: it was something that had so much belief that it was 'the High Seraph' that it became that, regardless of what it was before.

You still haven't shared the evidence that 'suggested that Ultima was originally a chunk of rock', but this approach would at least hold for that theory: this is a setting where faith has an effect over a long enough period of time, so it's not impossible that Ultima was just a big, radioactive rock that got deified over time.

I would argue that a big, radioactive rock doesn't make for an especially compelling 'arc villain' to build up to, though. If they're gonna do an amped-up version of Ultima, that's probably not gonna be the reveal.