I'd say if K3 were officially made, it would expound upon the fates of the crews from K1 and K2. Maybe introduce some new characters. Not sure what kind of "mission" it would have, though.
Who would there be to fight? Kunists? Remnants? Syndicates?
I'd imagine there might be loose ends from both games, maybe. It probably could include the Mandalorians and their journey, and HK-47 could make a cameo, or even star in his mission to get to the foundry since it was only mentioned... The restart of the jedi order.
Not sure anymore just what a K3 would constitute exactly. What is there to tell? It's all inconsequential ultimately, isn't it?
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