Some of the things that can be glimpsed in playing the early build only hint at what's to come. Specifically, you can venture off of the map defined for the tower defense levels and just sort of wander off into the woods. When you do so, you will find wolves and enemies that are just sort of out there minding their own business. For now, they are greyed out and can't be attacked or otherwise interacted with, but it appears that there will be "side quests" in the game when it is released. These side quests look like they will be short and more in the "random encounter" sort of category, unless there are to also be other maps that are somehow linked to from "doors" on the tower defense levels. At any rate, it was interesting to see these elements hanging around the edges of the level, and I do hope that these make it into the game when it's released.
Another thing available to a limited degree is the level editor. With what is available, you can create your own levels, from the design, to the paths, to the enemies and the waves. However, as far as I can tell, you can't alter the setting (for example, if you wanted your level to be in the woods), and there doesn't appear to be any way to raise or lower sections of the floors in user-created levels, while many of the levels in the game features stairs and differing heights. You can create levels and play them now, but the ability to share them via Steam is not yet available and, as such, you can't try out anyone else's levels, either.
While "buying-in" at the Early Access gives no guarantee of further development of the game, it is currently slated for a February 4th release (at the same price as the currently available Early Access version). Upon release, Neocore plans to include "way more levels" as well as additional Character Classes and the above mentioned user-submitted content.
There are multiplayer modes available - both cooperative and a competitive "Versus" mode. I haven't managed to get into either and actually play against anyone as of yet, but these modes are available in the Early Access period. I will put these modes through their paces and have a full report in my upcoming review after the game is released.