Easy solution to the gating problem / leveling up problem. Cut down on the filler "fetch" quests in the MSQ. You will still have the mandatory story dungeons. the only other solution is to continue as it is currently and only offer quest rewards and gil for completing MSQ. If you do not, then no matter what happens a new player trying to catch up to the current content will be over level for all of the 51+ quests, unless the Xp required to go from 50 to 51 is insanely high.

As it stands though, I expect from the length of time that this has been in development, that SE has probably thought of this already and has plans in place already to make it keep in line with normal character progression. All we can really do at this point is wait and see. Then judge based on what is actually put in place.