Long answer:
I think having the GMU would add life to the game. I've read through the back-and-forth and it's curious that it's said they can't ever enter due to the lore established in the Maiden Voyage or what certain characters have said, yet, as mentioned, the addition of the Sovereigns and GMU-turned-smuggler characters had to come from somewhere and we know where that somewhere is - outside the Shroud. Now, we can argue that their ships or fleets are too big, but here's the fact: introducing members of or former members of a faction outside the Shroud aren't isolated events. Why introduce them if they're not part of some bigger narrative? Why mentioned the factions involved if they're forever irrelevant?
The part that bugs me is that we see in the lore (game and books) that people have gone in and out of the Shroud, maps have been produced, and essentially all who populate the Sea of Thieves originated outside the Shroud, but this one faction is just somehow forever barred due to their ships for fleet being too big. It's as lazy as blaming the Sea of the Damned as the reason for the latest threats, especially when avoiding the implications such as saying the Veil provides a gateway while Ancient Megs arrive without it and it just so happens that the Veil appears to be missing some gems, gems that - if lore accurate - could be in the belly of the megs that allowed them entry. We can dig into the lore and try to make sense of things, but here's another fact: the writing as of late hasn't been the most coherent or plot-advancing. It's a slow crawl with no clear objective being spelled out that spells out the stakes outside of the basic hero vs villain conflict. Even S13 with the Blade and S16 were practically the same thing: go find stuff for Flameheart. Now we're about to get buffed forts that have yet to explain their purpose in advancing the narrative for Flameheart's obvious plan to gain power for an unseeable culmination.
Now, on to the GMU. If it were added, the first thing it would do is validate the Smuggler's League that's simply redundant in both quest-type and assets. Smuggling exists in the presence of an authority and its arms of enforcement. That doesn't exist in SoT thus smuggling is simply trying to get from A to B without being caught and/or defeated as is standard voyaging in a PvPvE game. The GMU would upset the power balance, forcing Flameheart to likely revamp his plans or even expedite them in the presence of a second powerful foe. Ramsey would also see the GMU as a threat and as unlikely as it is that the two would join forces, even if warring with each other again afterwards, the GMU would be attacked on multiple fronts which would require them to be a faction that's not just formidable, but catastrophic for pirates. How I theorize it could play out is we have one or even two seasons where the GMU is an existential threat and looks to gain pirate allies. They could have an emissary and those who fly it are immune to the NPC ships patrolling the seas. It'd be like a "pardon" like in Black Sails. Once that emissary is dropped, that player is an enemy again until reraised. This would also contrast with Reapers and even Athena since Rare has made it to where Athena will do battle with itself in HG.
Once the GMU is defeated, they can be pushed back to territory that remains theirs but heavily defended, like Port Merrick. There can be voyages and an sell-all spot like Reapers, but with Sovereigns and could sell slightly more than guild but less than Reapers, just to keep it interesting while also accounting for the convenience of a harpoon sell-all. Later, if Rare decides, they could have events where the GMU gets frisky again or even have a season where they're pushed of the the Sea of Thieves permanently as to show story progression of sorts. If Rare wanted to take it a step further, they could add player-controlled adventures where decisions permanently change the world, such as the permanence of the GMU's presence or banishment. And, to play devil's advocate with pirate profiles, they could have pirates pledge themselves to the GMU which would bring in the first iteration of choices with meaningful ramifications.
