The best course at this point in my opinion remains that it would the most workable to separate the necromancer/warlock class from the Summoner job title. It doesn't even necessarily have to be called a Necromancer or Warlock. Arcanist certainly doesn't have a precursor in FF lore, so you could conceivably come up with a whole new name - something like "Spirit-Eater" or "Soul Gatherer" or some variation on that theme.

Revamping the existing class isn't feasible or sensible, and you'll obviously have people like Silver who will certainly and reasonably object to it being irrevocably destroyed and replaced.

I am not sold on the idea that the new Summoner should be a pet class at all. In lore it was more of a mage, just with a spell-effect generated via the Summon. There's no reason it should have to be a Black Mage clone either. If anything I would like to see it using stances/combos corresponding to the primals (the how's and what's are open to debate).