I think that in order to be able to seriously talk about the Fury - Usyk fight as a competitive one, it is necessary that Usyk gain at least a little weight (can he, given that he has already moved to a new weight category for himself?). Now Usyk weighs 100 kg, and Fury 123 (or already more) - 23 kilograms of difference, in fact, they are in different weight categories.
He will lose his speed and probably his accuracy if he gains more weight than he can afford, he is ok with his current weight as long as Usyk maintains that speed and accuracy, he can fight anybody as he did with Joshua and beat him, heavyweight is the only division where there is no maximum weight limit, Usyk if ever will add more weight it will not be more that will hinder his speed.
That's why Usyk is really a cruiserweight, and not a pure heavyweight. Fury is very different breed of heavyweight though and same with Wilder, taller and heavier. And it is not advisable for Usyk to put in more weight, yes he might lose his mobility and speed which is a big factor in his win against AJ. So it's better to just used the same weight against Fury (if they fought), but it will take a lot to be able to beat the version of this Fury.