Seems that a fire attack against every non flying enemy creature in the game, plus wildfire imps, and the new synergies from Forged in Fire could be pretty fantastic.
I was working on this concept shortly before GenCon. I was going for the +1 to Demons with a fairly high count of Creatures. Other than having to use so much mana and two full actions to make the most use out of it, the main reasons I did not go with the new Warlock with Gate to Hell were that 1) Walls block the Gate's attack, and 2) because the attack does not affect conjurations. (Note that many of the Druid's spells are conjurations... even vine tokens are immune... although you would think that this attack would be great against her. Also, a number of the War conjurations are Flame +, but also are not affected.)
I swung the other way with the new Warlock now. I am thinking that fire attack spells with the Imp familiar and a few of the bigger Demons might be worth a try. The trouble with Demons, though, seems to be that you need to mix the ones that are Flame Immune with the ones that have Piercing. I think you have to take some of both and use the ones that make more sense at the time. This means that you are using extra spell book points for the flexibility (if it matters to you).