I think given SoT's lifespan and seeming lack of inspiration lately; I think it's time to introduce the GMU.
Implement them as an enemy faction that forces the Servants and Guardians to reluctantly ally, move the story foreward, shake up the sandbox and storyline.
I think they saved Flameheart and the GMU too long, personally. The last 2 years of SoT has been mostly stangant in all honesty, being about 75% new ways to track doing the same old activities.
Flameheart being implemented but not really impacting the game at all is ... startling.
