Considering it's nearly impossible to predict when a more major update of the AppleTV is coming, and the device cost is fairly low (especially if you buy a refurb), I'd just buy one now if you're in the market for it.
Other than further feature updates via software, I think the only likely further development is supporting third-party apps. Even if Apple does add third-party apps to the AppleTV at some point, it's likely that current models will be supported, even if a new model is released at the same time (unless it's a totally new product, like an actual TV). So I don't think you'd be missing out on anything by having the current model.