I believe they'll allow it if:
1. The item you want to use to remodel, is equip-able by the class you're on OR the gear you want to remodel is for the same class(es)/job(s) as the gear used to remodel. I say this since it depends on how they'll go about it in the end: vanity as slots (a-la Rift) or item alteration (a-la WoW)
2. The item used to remodel has stats. I don't think they'll allow "bikinis" for example that are casual wear to be used. All previous event gear that was, in my opinion, added for future use in the vanity system (such as the Lightning's Fates gear), had some stats.
3. Whether the item you're remodelling has at least the same character level requirement as the item used to remodel. (NOT ilvl, ilvl would play no role)
Of course in the future (or from the start ? avoid the stat check?) they could add items that are specifically designed to remodel the usual sets, such as pure vanity sets with no combat stats but with a "stat" called "Usable for vanity" or something. Items that were not designed with combat or crafting/gathering in mind (bikinis, Santa costumes, reindeer costumes, etc) would not have the "Usable for remodeling" stat.
Also they could possibly add another stat to an item called "Remodel-able" that indicates whether an item can be used in the vanity system (made to look like another piece of gear). If they wish certain items to be set in stone in terms of looks... (thou I am not sure why they would)
They could think this out very well, without going overboard in restrictions while still maintaining a large degree of customization.
Personally I do not care for using bikinis in DOW/DOM/DOL/DOH. Using heavy Paladin gear on my Black Mage ? Nope ! Feels very out of place and awkward. If they had a "Battle Mage" or something that can wear "plate" like items then I'd be ok with it, as it stands however, BLM is a "clothie" in my eyes as they're sometimes called ^^".
You could give the example of the plate bikinis in or caster bikinis from Darkhold, but I beg to differ, those were specifically designed for combat, even if they're obviously offer little to no protection.. but it's intended. Summer bikinis were not intended for combat.
Sure some people may argue that there should be no restrictions but I think it would be detrimental to the game's overall feeling and the immersion (yes, immersion) if there wouldn't be some rules along the lines of what I described above.
Anyway, my 3.141592653... cents ?