In addition to playing a chord (all strings open is a chord) and picking some sort of pattern over it,
you can also insert scales and "runs" (short pieces of scales) into rolls.
To begin with, here is the G-scale in tablature.
These are the notes "do-re-mi...", the Major Scale, for the key of G.
They are the notes you'll use most often when playing songs in the key of G.
And here the scale is extended down to the lowest note on the banjo and uses the 5th string for the final note.
Play these notes up and down the scale to get your fingers used to the positions.
And here is the scale showing the tuning notes—notes that are the same on two different strings, one of which is open.
Here are some basic forward roll modifications you can try.
These examples are using 3 of the 8 notes of the roll for melody: T M T I M T I M
In this line, the three examples go up one note of the scale for the final melody note.
In this next line, the three examples go down one note of the scale for the final melody note.
Because you can't do a complete roll (of this type) playing the melody notes on the 1st string,
in the first exercise I move the high D down to its tuning note on the 2nd string.
You can, of course, change notes for each of the melody notes of the roll (or for any of the other notes, too, but it's easier to take small steps when trying to understand what's going on).
This next line has only two exercises because they tend to imply an ending (in the key of G).
You should practice just the roll by itself also in order to be able to use it other places.
If you don't want to end up being a tab/cd/sheet music drone, you should start right now experimenting with all the possible changes just with this basic forward roll over the basic G scale. For instance, change notes for each of the three melody notes in the roll:
2nd string: 0 1 0 or 0 1 3 or even 0 1 2 (which isn't actually the G scale). Go down too: 3 1 0
3rd string: 0 2 0 or 2 0 2 or even 2 3 4
4th string: 0 2 0 or 2 4 2 or 0 0 4 or 4 0 4
These are the most basic changes you can do. In addition, you can add chords (start with C and D7).
For example, on the 4th string when playing the 0 2 0 melody, make a C chord and hold it the entire time except for lifting the one finger on and off the 4th string.
Fool around and see what works and what doesn't.
You can also add in special effects in different places.
Here are three examples of adding the same slide on the same string three different places in the roll.