None of the above.
If they need a larger aircraft then it should be the A321 because it is common type rated with the A320/A319/A318.
The 767 is better suite for longer routes. Frontier doesn't have too many routes over about 5 hours. The A330 is suite for intercontinental flights. Frontier doesn't really have any of those. The 757 is about the size of the A321 and if they got an aircraft of that size then, as mentioned above, they should go with the A321.
My understanding (and correct me if I'm wrong) is that while the A321 can fit more passengers, its maximum range shorter than the A320, A319, and A318 (the latter they removed from their fleet a few years ago).
B757? not going to get any that don't have some miles on them, and the B739 is about the same size of it as well, and why go to that when they could keep commonality with the A321 (minus the range issue above). Anything above that is overkill and won't fit into their business model.
I don't know what would work best for them outside of what they already have now. Ejets work for them when they feed into their main network. Increasing capacity is a tough call because of how their model is set up. They could benefit from sharklets, but they would still be using the same fleet.
In the end, adding something uncommon to their fleet would be a huge mistake.
I would also have to say 'none of the above'.