I'm grouping these together because either A) I don't know which god I'll be doing anything with or B) the models aren't god-specific.
As soon as I add some more mobile units and/or tank killing units, I can drop one of the Bloodthirsters. I could probably drop both, but I'd like to keep at least one: what's Khorne without a Bloodthirster?
That means swapping him (or is it her?) out for one or two Heralds. I haven't done much thinking about what type of Herald I'd take, but... I can't see myself bothering with a Herald of Nurgle, but I can imagine a place for Heralds of Khorne, Slaanesh and/or Tzeench.
I'm pretty sure that I'd mount them in a chariot, regardless, which means any conversion would be driven by 1) how I convert the Troop-level Daemon (and it him up) and 2) how I convert the Elite-level Daemon pulling the chariot.
Soul Grinders are Soul Grinders. Done.
Daemon Princes are only a little trickier. For the time being, I think I can run the ones I'm in the process of painting as whatever type is called for. Longer-term, if I settle on a build I really like, I can convert up something new: almost certainly based on the Minotaur kit. I love those models.