The module should provide an additional option for product attributes.
When using product attributes with price modifiers, there is currently no way to decide whether the additional costs of a specific attribute be added to the setup fee or to the subscription fee, which for example makes selling recurring services with various options impossible.
As a more general concept, it would also be very interesting that a subscription would not be entirely bound to a product, but more to an order (which would I suppose allow for the above feature).
A simple use case:
I want to sell an online service as 3 different plans (bronze, silver and gold). With the current features, I am unable to offer a plan upgrade to my customers during the subscription time which makes it very limited.
When manually stopping a subscription, there could also be some kind of reimbursement feature which would at least allow for stopping a subscription, reimbursing the customer for unused service time, and let him subscribe to another plan.
We are planning to add support for product options/variants for X-Payments Subscriptions module for X-Cart 5.