Hey! I've been hiding in the shadows for a while, but here I am!
I, personally, think that chances are greatest that this will be a coaster. What type, I don't have much of a guess. Aside from odds, my top picks (currently) for what this attraction will be, based on personal preference, are as follows:
Log Flume. Whether the old flume stays or goes, I would love to see a highly themed/scenic log flume come to Rivertown. Otherwise, whether they use a great deal of theming or not, a longer, more interesting/exciting log flume (perhaps even including more than one drop) sounds like a great way to traverse the woods of Rivertown.
Wooden Coaster. Yes, The Beast is close to this location, but it seems to me like a woodie would "feel" right in this location. I believe it was mentioned earlier in this thread that a woodie could fit the theme of the area well. Besides, woodies just have a certain charm to them, do they not? Also, I think it would be fun to have a woodie that makes use of some modern woodie elements. (See, Outlaw Run)
Dark Ride. I think this is probably the least likely given the placement of markings, etc., as has been pointed out regarding flat rides and a dark ride, if I'm not mistaken. I think Rivertown has the potential to provide a good environment in which to set a dark ride, but I don't think this attraction will be a dark (or, not entirely a dark--I could, perhaps, more easily see a log flume or coaster that is partially enclosed, providing an indoor/outdoor themed experience.)