If you could learn one new system what would it be

There's a huge amount in variation to be found in Judo ground fighting skill, both at the school and the individual level.

There are Judo black belts who have newaza equal to a BJJ one-stripe white belt. There are Judo black belts who have newaza equal to a BJJ blue belt. There are Judo black belts who have newaza equal to a BJJ purple belt. There are Judo black belts who have newaza equal to a BJJ brown belt. There are even Judo black belts who have newaza equal to a BJJ black belt (although these are unusual).

The Judo black belt newaza = BJJ blue belt might hold as some sort of statistical average if you sampled enough Judoka, but I couldn't say for sure. Too many of my Judo friends have also trained BJJ, which makes it harder for me to see the statistical big picture. I will say that the specific newaza skills will typically be different. A Judoka who is skilled at groundwork will typically be good at top control, pinning, attacking the turtle, hunting for a very quick choke or armbar, and perhaps passing the guard if that can be done quickly. A BJJ practitioner will have a broader range of groundwork skills including sweeps and submissions from the guard, a wider range of guard passing options, setting up attacks that take longer, various submissions which are illegal in Judo competition (such as leglocks), and possibly using/defending against strikes on the ground.