So what happens if you're a harmonica player in a band and the song that's being covered doesn't have any harmonica in the original version?
Where do you go for ideas?
Well in this video, I want to look at Led Zeppelin's Ramble On from a harmonica player's perspective and demonstrate what I would do with a song that doesn't normally have a harmonica part.

0:56 - Harmonica Key

Ramble On is in the key of E. This song is heavily blues influenced so we want to play in second position where we can access the notes from the Blues scale.
In order to do this we need to go up a fourth from our key of E which brings us to A.
Using an A harmonica while playing blues in E puts us in second position.
Now the full E7 chord is available at -2-3-4-5 as well as the Blues scale at:
-2 -3b 4 -4b -4 -5 6

1:52 - Bass Line

There's a great bass line played by John Paul Jones that we can lift from.
Here's the tabs:
Bass Riff
-2 -4
4 -6 6 -3
-2 -7 -6 6 5 -4 4 -3 4 -4 4 -3
You can have some variations to that but that's the basic idea.

4:52 - Chorus

Time to add some tasty fills that don't overwhelm the vocals.
Pick your spaces and utilize Blues Scale licks.
Blues Scale Tabs: -2 -3b 4 -4b -4 -5 6

Sample Lick 1: -4b -4
Sample Lick 2: -5 5 -4 4 -3b 3 3 2 3

7:50 Guitar Solo Harmonies

-4-5
45 -4-5 45 -4-5 45 -4-5 56
-4-5 45 -4-5

10:45 Guitar Solo Fill After Chorus

Lead
5 -4 4 -3 3
5 -4 4 -3 3
5 -4 4
5 -4 4 -3 3
4 -3 4 -3 4

Harmony
6 -5 5 -4 4
6 -5 5 -4 4
6 -5 5
6 -5 5 -4 4
5 -4 5 -4 5

RECAP

That should give you some ideas on how to come up with parts for a song that doesn't have a harmonica in the original recording.
Choose the correct key and position.
Find some space for your creativity.
Incorporate melodies and rhythms already used in the song. 

Do you have a favourite melody that you would like tabbed out and demonstrated on harmonica?
Let me know.

In the meantime, stay in tune. Peace.
George