I wonder, if the Russian states never unified, if Poland-Lithuania might replace Russia as a trans-Siberian empire.
It might have, but you'd still have the mix of ethnic Russians and Ukrainians colonising Siberia. Probably way more Poles in the Ukrainian steppe, and more Polonised Ukraine (we'll just assume that anything like Chmielnicki's rebellion is either butterflied or quickly crushed if we want maximum Poland-Lithuania).
I bet at some point this Poland might go west, too--a Poland-Lithuania with modern-day Poland's western border seems like a natural ambition, of course ignoring the fact that in many of those places Poles (or any Slavs) hadn't been present in any noteworthy amount since the Middle Ages.
This.
However, without the Russian threat, the Grand Duchy of Lithuania would have likely stayed in personal union with the Kingdom of Poland for much longer (they were forced to transform the personal union into a loose federation under Polish leadership in 1569 after Ivan the Terrible's armies took large bits out of the Grand Duchy). With Russian provinces of the GDL growing relatively fast, it would be likely to grow stronger than its Polish partner at some point in the 17th century, and then probably try to enact an alt-Lublin Union, creating a Lithuania-Poland (probably with its capital in Vilnius) instead of OTL's Poland-Lithuania.
Since this Lithuania would be dominated by Russians and other Slavs, wouldn't the capital be in Kiev or Moscow or Minsk or any other major Slavic city? Of course, Vilnius was pretty Slavicised, since well into the 19th century Lithuanians were a small minority there (and even outside the city it had very large amounts of Poles).