For instance, if I were fusing Raag Malkauns, a pentatonic raaga with notes tonic+minor 3rd+4th+minor 6th+minor 7th, I would do so with a B tonic and fuse it with a celtic piece in G major pentatonic. This would make both pentatonics relative. But the FEELING of Malkauns is particularly problematical for fusion with Celtic IMO. A better relative pentatonic would be Raag Durga (Bilaawal's Durga) with D tonic both in feeling and the mutuality of resolution (ie in this particular case: Celtic G tonic with the Indic D tonic - power chord relationship/primary harmonic).
actually a viable modal relative approach with Raag Malkauns is with Raag Dhani. The Former is the 3rd pentatonic and the latter the 5th pentonic mode in the major pentatonic scale.
So if we play "Bonnie White Horseman" with E tonic we have the scale of Raag Dhani and using that scales 5th (the B) as a tonic we will get the scale of Raag Malkauns. Both are minor gapped modes.
It really works quite fantastically.