First of all, the product looks great. I'm about to implement it on my system, even though it only allows for four subscription types. I have four types right now, but I also plan on expanding, utilizing a digital product for separate payment for download. More than four types or unlimited payment structures would be great.
On that note, another feature I'd like to see is an option to lock in a subscriber for a Year but only charging them monthly, where say they "unsubscribe" early, they are charged a fee.
My third want is to have work with groups/roles beyond the WP default roles. Whether that's something created within S2 itself or maybe merging a bit with another product like Role Scoper or something one of you guys use that might be a little less bloated.
Maybe S2 does some of this already, and I just haven't explored it yet. However, I like the potential of this product, especially since it's an active group seemingly committed to making this product better.