We'd love to get there.* Encourage your friends to pick up the game!
I'd suggest an add-on first.
No story line needed - just using the existing towers and aliens, create a patch with another 10 or 20 maps. Goes easily (?) and quickly (!), and you can sell it for a nice tidy sum ;-)
And then look at all the feedback you collected, and see if you want to create DG II, with a new story line (such as it is), and more functionality (which, in My Most Unhumbly Opinion, you don't need - the simplicity is perfect) .
And, obviously, sell it yourself and directly, making a higher percentage off each sale, and doing without this DRM nonsense