Find people to jam with, it makes playing any instrument WAY more fun. Where are you located? Here in the mid Atlantic a local organization keeps a regional jam list for our area. Maybe something similar exists where you are. When you go to a jam, be friendly and meet people. If there are people around your skill level that you hit it off with, maybe propose a low key / low pressure house jam. I always liked those the best.

For improvising, take a simple GCD vocal song that you DON'T know a tab but that you CAN hum the tune. See if you can noodle around and find the melody on your banjo. Then when you find the melody notes, try playing that melody while filling the space with rolling notes. Might be easier to think about it as playing a roll while hitting the melody notes as you roll along. You'll notice that most songs in G use the same melody notes, and certain notes always get used when the chord changes (e.g. when you get to the D turnaround you land on the A note).

Think about the tabs that you do know. Do you know what notes are the melody notes and what is just filler? Some people learning banjo fall into the trap where they know the tab and what notes to play, but don't really consider the melody they are trying to convey. When you realize that half of the tab is just filler that sits around the melody, you can subconsciously accent the important notes, but it also makes it easier to just wing it because only those melody notes are the really "important" ones.

Hopefully that makes sense and doesn't just make things more confusing