Personally, I'm kind of halfway expecting a twist with Gear 5. I think Oda might end up revealing that the Nika powers themselves are a threat to the world. So, things might get more serious, but the stakes might come from Luffy struggling to control the Nika powers, rather than any opponent he faces.
We've seen in Elbaph that Luffy seems to be able to stay in Nika form longer than he used to be able to. Now, this could just be Oda being lazy, or it could be a plot point. It's possible that the Nika powers get stronger the more Luffy uses them.
So, it's not impossible to imagine that the Nika powers could eventually grow so strong that Luffy has trouble containing them. They might be the equivalent to a nuke, with the powers going critical if they reach past a certain threshold. Maybe he feels so much "Joy" that he can no longer tell friend from foe. Maybe he ends up warping reality so much that it just doesn't turn back, like how Aokigi and Akainu's battle permanently changed Punk Hazard with their fight.
But, I think it's telling that the Gorosei and Imu seem legitimately concerned about the Nika powers. This could just be because they're powerful, but it could also be because they are dangerous. See Chapter 1111, where Warcury asks "Do you have any idea who that guy is?!" I think they're actually worried about what those powers are capable of doing.
So, I just think it's possible that Gear 5 becomes more dramatic with Luffy dealing with his "madness", rather than because of a dramatic fight with a strong opponent. Luffy might end up accidentally crossing a line, and everyone might have to keep a eye on him. And freeing Loki does feel like a potential "crisis point" where that kind of plot point could start to come into focus.