Yes exactly. The beauty of this idea is that it only skips 2 dungeons, those being snowcloak and keeper of the lake (it also skips HM primals, but they are hardly content anymore considering how easy the ex versions are). What this would do is allow you to access ishgarde as soon as you beat the praetorium. My vision is that at any point during the 2.1-2.55 MSQ, you can go to the guard on the steps of faith and request to skip to the expansion. It will then play a summary video (as I pointed out, far easier to do than a summary video for 2.0-3.55) and allow you entry to ishgarde
In hindsight, you are right. The one difficulty with this change is that class quests will have to stay within the expansion area (as that is a huge change that is not worth it). It's a difficult problem that is not easily fixed, and is best learned from in SB.
This is a bit of a strange one as a lot of dungeons have MSQ context, Keeper of the lake especially. What my proposed solution does is it allows people to pick and choose which bits of story they do, while still having the complete experience. Some dungeons will unlock the way you propose under my suggestions (specifically snowcloak, keeper of the lake, anti-tower, and sohr kai), with Xephatol and Baelsars Wall being added later when they are not relevant to the current expansion (being the epilogue to 4.0 that they are)
Actually the only one that involves changing hooks is the job one, and even I'm not fully sold on it (see above). All of the others actually function the same as the story jump potion, except it works much smarter. What my solution involves is skipping story content that does not involve the levelling process, and allowing players choose which parts of the story they want to skip during patch content, where there is a lot of filler to make up a patches worth of content. Unlike the jump potion my method A) doesn't cost money, B) allows stormblood's story to still make sense C) allows players to get through the story much faster D) still gives players the levelling process to learn their jobs, and E) contratry to what you have said, is feasible with the information that we currently know about the jump potion.
Yes it will take some work (not nearly as much as you are proposing though), but I would expect something as important as this to take resources. This would be one of the last majorly requested QoL change to be fixed.