Demons, Heaven, Hell and parallel universes

[No_Nick_Needed]

The whole thing started with the Milala and was only later corrupted and refined by Preservation, but even if we ignroe this and just look at real fusion itself, it's original intended purpose still does not make it inherently evil and impossible to use for good.

The Garlean war machina were developed to conquer and subjugate the world and yet we and others used some of them to fight back agains the Garleans, and later the Garleans used their makina to safe people from blasphemies.
There's also a good chance that airships in general were first and foremost developed as military vessels by Garlemald and yet we all love the Enterprise and nobody would consider her evil.
We used parts of Dalamud to upgrade the engines of the Ragnarok, so we could get to Ultima Thule.
Omega was designed to wipe out other civilizations, and yet we didn't only use it fight Shinryuu to a stand-still, but eventually even managed to turn it around into... well, at least a neutral, non-hostile party, if not exactly a friend or ally.

Again I did not decide anything, I asked a question. Is is possible? Is there a scenario, where it makes sense?
Obviously it should not be done, just because it can be done and would only be a last resort, if all attempt to stablilize the shard in question fail.

There's also the Void and the First to consider. We already had one instance of pulling light aether from one to send to the other, so we already have a kind of proof of concept of dumping one's excess aether into the other is possible and having apparently a positive effect on both. Realm fusion might be a way to do this in a way that more directly connects the two reflections, without the complication of having to utilize the Tower of Babil, the Crystal Tower and the seals that once held Zodiark, to keep an accidental void-sent incursion from happening.
Again, I'm not necessarily saying it can or should be done, but I think it's worth thinking about, or at least fun to speculate a little bit.

[MikkoAkure]

Again I did not decide anything, I asked a question. Is is possible? Is there a scenario, where it makes sense?
Obviously it should not be done, just because it can be done and would only be a last resort, if all attempt to stablilize the shard in question fail.

With the Ascians gone and the past 2 world hopper threats put down, there's no scenario where needing to stabilize a shard or escape from a calamity would actually occur unless the writers invent something new and I'd hope they would try not to retread the same plotlines yet again.

Rejoinings are an act of destruction and even if you did the theoretical "safe" route, teleporting whole populations of people into the Source causes more problems if Alexandria is anything to go by.

[Cilia]

... [T]eleporting whole populations of people into the Source causes more problems if Alexandria is anything to go by.

The real problem with interdimensional fusion isn't really the displacement of people, but that it brings along the environment from the area fused. Theoretically if you fused an area from a more environmentally healthy and aetherically balanced world (as opposed to the lightning oversaturated hellscape the Ninth apparently became) it wouldn't have the sort of impact Alexandria did on Yyasulani - you'd still wind up with a load of temporo-spatially displaced people and alien structures, which would still wreak environmental chaos on wherever they fused onto, but the people themselves aren't the problem.

Still not a great idea.

[Cleretic]

The real problem with interdimensional fusion isn't really the displacement of people, but that it brings along the environment from the area fused. Theoretically if you fused an area from a more environmentally healthy and aetherically balanced world (as opposed to the lightning oversaturated hellscape the Ninth apparently became) it wouldn't have the sort of impact Alexandria did on Yyasulani - you'd still wind up with a load of temporo-spatially displaced people and alien structures, which would still wreak environmental chaos on wherever they fused onto, but the people themselves aren't the problem.

Still not a great idea.

Also worth saying, fusing locations still leads to a net negative.

Like, let's say just as an example you're fusing Il Mheg onto Coerthas; that's probably the best single one you do ecologically, Il Mheg's one of the healthiest parts of Norvrandt, while Coerthas is still dealing with a winter that may well be permanent. But when you do that, you basically overwrite Coerthas with Il Mheg; there's no longer two distinct locations that people live in and make the best of, there's now only one. The same amount of people, now trying to share less resources.

And that one's without factoring in the populations, which I agree with Mikko is a big problem especially in that hypothetical; let's be honest, fusing Il Mheg and Coerthas just creates the most singularly horrifying war that Eorzea's ever seen.

[PorxiesRCute]

The whole thing started with the Milala and was only later corrupted and refined by Preservation

This is such a good point. Azem's Sippy Cup doesn't even necessarily have to do worldmerging (although it might be beneficial for everyone if a small part of the First was merged with the Thirteenth and vice versa...). Also, it's pretty blatantly set up for the WOL to use the thing to travel to other worlds at some point in the future, maybe in 8.0.