I kinda think that Bowser's old moveset works on Giga Bowser, He'd look terribly goofy throwing out dropkicks and the new N-air animation. However, they might redesign either the model to make it more suitable for the new moveset or they may tweak the animations to give them more flare. However, I most likely will not be using final smashes outside of maybe getting a look at what they do or in some silly matches if I happen to visit the for fun mode to goof around.
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S_B
Unfortunately while it does look like a fortress to Fsmash, it's actually just Bowser pivoting during his new Fsmash charge animation. It looks like a fortress because of how he hunkers down in the animation and the fact that the pivot makes it look like he's spinning like in a fortress.