The big problem is what everyone has mentioned, that access to HW content is locked behind the msq. The issues just snowball from there.

My recommendation to address it would be to allow players access into Ishgard to start the HW msq as soon as they hit 50, regardless of where they are at with the ARR msq. They could always backtrack later to do ARR, of which they would be inclined to do at some point anyway when they start realizing some of the lv.50 dungeons might still locked. But at least it would be the player's choice in managing how to tackle it.

Change a few of the first msq scenes to adjust for the transition, and/or any other story tidbits that might reference ARR, and everyone is good to go.