What protection are you after that your H&G doesn't offer?
As for leather jackets, which won't offer the same water resistance, you'll find jackets meant for two-pieces are shorter. So if you JUST want the jacket, and you're not planning on zipping up to your trousers, make sure it's long enough to cover your lower back when lent over.
Separately sold back protectors are often recommended rather than the supplied one in jackets. You can get chest protectors as well. You'll want decent elbow/forearm and shoulder protection.
Check out the threads in the
riding gear forum.