I haven't played WoM yet, but, if I had to say, I think there are three broad sets of options:
1. Market it as a reboot, calling it Elemental: Fallen Enchantress.
2. Market it as a sequel, calling it Elemental 2: Fallen Enchantress.
3. Market it as a new game [secretly in the same universe], calling it Fallen Enchantress.
Personally, I think you should pick either 2 or 3. Why? Although 1. works if you know a bit about it, it is otherwise hard to distinguish from an expansion or expandalone. You probably don't want that. At all. The subset of people who are not keeping close tabs on this project but who would potentially be interested are probably going to be turned off greatly by the chance that this is just an Elemental expansion or expandalone. Also, they may read reviews of War of Magic and assume it's for the new game. They might casually ignore the generic fantasy subtitles.
If you market it as a sequel or as a new game, that's less likely to happen. However, the choice here is whether you think it is best thought of as a sequel or as a new game. If you really think it's that different, you may be best separating it entirely. You may even reintroduce Elemental later, but just sort of skip it for this entry. If people know it's a -sequel- to a bad game, they might still dismiss it, but they're more likely to assume bigger changes for a sequel.
Honestly, the worst outcome is it goes out and people assume it's an expansion or expandalone. I highly suggest you combat this. I also suggest reaching out to RPS, joystiq, kotaku, PCGamer, Destructoid, etc. as you get a bit closer to launch. The more people writing articles with titles like 'FALLEN ENCHANTRESS IS A WHOLE NEW GAME" the better. Alternatively: "STARDOCK PROMISES TO MAKE THINGS RIGHT."