BLM could use some button consolidation

[PyurBlue]

As for Scathe/Xenoglossy. You can use Scathe while Xenoglossy is available... but you’d never want to.

I can't agree with this. Scathe doesn't cost Polyglot so there are definitely times I'd rather use Scathe than Xenoglossy. Using Scathe is far better against weak or underleveled enemies, or enemies with a fraction of HP. Being forced to use Xeno in those situations would be a complete waste and feel horrible.

[Seraphor]

I can't agree with this. Scathe doesn't cost Polyglot so there are definitely times I'd rather use Scathe than Xenoglossy. Using Scathe is far better against weak or underleveled enemies, or enemies with a fraction of HP. Being forced to use Xeno in those situations would be a complete waste and feel horrible.

If a Scathe would kill them, then so would any other attack from your party members. Just wait 1 second.
Might be a niche use in dungeons, but nothing essential and there's no pressure to save those couple seconds.
In a trial/raid setting, absolutely useless.

[Archwizard]

Most propositions are not realistics. Lay line and between the lines are 2 coldown, even if between the line is a short. one. You can scathe without condition, even if xenoglossy is up. Etc.

For combine 2 spell you have to create a capacity which change depending if you are under condition like the pvp talents. Exemple ice 4 become fire 4.

Thats say you can create simple macro for that, so players keep the choice to keep each spell on the keyboard or not.

As Seraphor stated, the capacity for many of these suggestions already exists in-game, particularly the dynamic skill replacement which is used by several other jobs like RDM finishers, SMN's Ruin, DNC's steps, AST's cards, etc; such methods simply haven't been deployed for BLM yet, even though they would be realistically applicable.
Meanwhile a majority of these suggestions are about combining buttons that are either mutually exclusive (like Fire IV and Blizzard IV, or Despair and Umbral Soul) or dependent upon one another (like BtL with Ley Lines, or the IV spells with Enochian), or outright trimming skills we already don't use because we have significantly better versions (the II spells, or Scathe).

transpose could turn some spells into aoe version and vice versa?
or a skill similar with the execution of Transpose

Transpose is fine as it is, honestly. It's the only oGCD method to keep Enochian going, on top of being the only reliable instant means to do so without popping CDs until we get Umbral Soul. Even if it's an absolute last resort at 80, it's good to have such a tool on hand.

[PyurBlue]

If a Scathe would kill them, then so would any other attack from your party members. Just wait 1 second.
Might be a niche use in dungeons, but nothing essential and there's no pressure to save those couple seconds.
In a trial/raid setting, absolutely useless.

In most cases sure, someone else will just kill it, but there might be times when you're the only one who notices or are the only one in position to attack. These are really niche situations but that's just the nature of Scathe. It's not normally worth casting. If you're just looking at it from a standard rotation point of view, then Scathe is useless (although in that case I'd just say take if off your bar instead of consolidating it with something else since at least you would have it as an option just in case).

Bringing up dungeons Scathe also useful for adds that can't be damaged but require attacking anyway. Adds like the mud pie in Saint Mocianne Hard. That particular example is a level 70 dungeon, but if the mechanic gets reused at 80, I'd much rather pay 1 hotbar slot than Polyglot. This is again something that you might count on your party for, but things don't always go as intended.

[Casino]

There are and will continu to exist some target posible to OS with scathe like the ice block of the myst dragon.