A riff is a short pattern of notes (usually one or two bars long) which repeats throughout a verse or section of a song. A riff can also be varied to fit a chord progression. The use of riffs is common in many styles of music including Rock, Metal, Blues, Jazz and Funk. Many of the examples you have already learnt are riffs. Here is another example. There are also several more on the following pages.
Here are some more riffs. Learn them and then make up some of your own based on the ideas presented here. Don't worry about the notes too much, just experiment until you find something that sounds good and then memorize it.
Here is a rhythm part which makes use of power chords, major chords and double note runs. Although this example is in the key of A, it is not strictly in a major or minor key, but makes use of elements of both. This is common in many musical styles. Whatever sounds good is ultimately the best thing to play, regardless of what scale or chord it comes from. Experiment with all the sounds you have learnt and have fun with them.