This won't work. Leaving FATEs the way they are and buffing everything else would definitely give people more choices about how they choose to level. However, it would also double (or triple) the speed at which players level. As it is right now an average player can get from 1 to 50 in a week of spamming FATES. Increasing experience gain on guild leves, guildhests, and dungeons could potentially shorten that time to a few days.

All they need to do is spread the experience more evenly between all of the activities. Leveling speed won't be effected and people will have more choices. It's a win/win.