Mostly the pbs. I figured that ghoul and vamp or sire and childe would both allow for a built in level of thick as theives, and definitely the blunt and kind of kickass energy of Margo in latter seasons will probably inform aspects of personality, but there is by no means an assumption that they be recreations of Elliot and Margo. Initially on reading on City of Shadows what their bloodlines were, the idea of a Mekhet Khaibit with a Summer Bishil pb that actually came from around Egypt was what came to mind, and I thought it would kickass to have an Elliot of sorts to be thick as theives with (ie more the dynamic with Elliot than a carbon copy of his persona).
Invictus is mostly because I've seen Mekhet do really well with them reliably and also, well, established as 'old money' type power brokers. Now of course as neither char has been made there is inifinite room for being flexible on clan and such, I could also see Ventrue working well, or Daeva, though less so with Gangrel or Nosferatu.