Sure, here's an expanded version of the riff with some added variation:
``` 
F (quarter-note) -> F# (eighth-note) -> F (eighth-note) -> A, C, C# (quarter-note) -> G# (quarter-note) ->
F (quarter-note) -> F# (eighth-note) -> F (eighth-note) -> D, A (half-note) ->
F (quarter-note) -> F# (eighth-note) -> F (eighth-note) -> A, C, C# (quarter-note) -> A Octave Up (quarter-note) ->
F (quarter-note) -> F# (eighth-note) -> F (eighth-note) -> D, Ab (half-note) ->
F (quarter-note) -> F# (eighth-note) -> F (eighth-note) -> A, C, C# (quarter-note) -> G# (quarter-note) ->
F (quarter-note) -> F# (eighth-note) -> F (eighth-note) -> D, A (half-note) ->
F (quarter-note) -> F# (eighth-note) -> F (eighth-note) -> A, C, C# (quarter-note) -> A Octave Up (quarter-note) ->
F (quarter-note) -> F# (eighth-note) -> F (eighth-note) -> D, Ab (half-note) ->
G# (quarter-note) -> A (quarter-note) -> C (quarter-note) -> F# (quarter-note) -> F (half-note) ->
F (quarter-note) -> F# (eighth-note) -> F (eighth-note) -> A, C, C# (quarter-note) -> G# (quarter-note) ->
F (quarter-note) -> F# (eighth-note) -> F (eighth-note) -> D, A (half-note) ->
F (quarter-note) -> F# (eighth-note) -> F (eighth-note) -> A, C, C# (quarter-note) -> A Octave Up (quarter-note) ->
F (quarter-note) -> F# (eighth-note) -> F (eighth-note) -> D, Ab (half-note) ->
G# (quarter-note) -> A (quarter-note) -> C (quarter-note) -> F# (quarter-note) -> F (half-note)
``` 
This version repeats the first four bars and adds a second variation that explores the lower register of the guitar. Feel free to modify and experiment with this riff to create your own unique sound.
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