"will it have a more rugged design"
-> Can you be more specific?

I assume it might have more durable material. There's a very different question on "if the iPhone will have a design aesthetic similar to the Apple Watch Ultra", which has a distinctive style.

I'd personally expect the iPhone Ultra, if it is released, to be more durable than the iPhone Pro (~70% chance), but I wouldn't expect it to have a design aesthetic in the vein of the Watch Ultra (~20% chance).