- Gold - The transmog armor sets and teleport spells will be unobtainable and remain character specific. The Gold guild achievement that rewards a pet will be unobtainable.
Join a guild that already has it unlocked? I imagine they keep it as a reward. If not in WoD, then join one of those guilds now, a friends guild, get a social invite, whatever. Getting the rep is super quick, especially if you already have revered or exalted on that character, as you don't lose all rep joining a new guild.
Rep means little, so just tell your guild you'll be back shortly and explain your plan (or use an alt)