There are a
few rough blueprints floating around like the BB1936 (AKA UP.41) that WG could use as basis for vessels above T8 (at least for the battleships; not too sure for the cruiser line, which would stop at T8 Zara). They also should have more than a little documentation lying around in
Russia considering the Ansaldo company had sold a few BB designs for the VMF (if you look at some of the Russian cruisers, you can definitely see the Italian influence right there - especially in the guns and conn towers) that were also considered for the Regia Marina, and more than a couple blueprints had been sent over to Moscow before war broke out. Still, a hypothetical Italian line would have more "actually built and entered in service" ships than the upcoming Russian BB line.
Besides...if I may be a bit snarky here...if you look back at Zao, Hindenburg, Roon or
Vladivostok, WG has no qualms cooking a design from the ground up, using vague cues from preceding vessels. Heck, even the Georgia is based on a very vague Iowa precursor design that briefly toyed with the idea of 457mm guns (and, yes, a single 18'' gun was actually built and test-fired by the USN) before going back to 406mm.
That said, I know they're pandering to their non-negligible Russian playerbase and this is merely a business-oriented decision, but the history aficionado deep inside of me chafes at the continued lack of (regular) ITA ships.