Hi Julianus,
– can i use a custom gateway for payments?
All of the source code is available, so you can certainly make any customizations, including adding a custom payment gateway.
– can i integrate a different login system (based on shibboleth)?
s2Member uses the WordPress login system. If you wish to use a different login system, you’ll need to make the necessary customizations yourself. As previously mentioned, all the source code is available for you to modify as needed.
– can the subscription to expire every january first?
Yes, you can create subscriptions for a specific length of time. You can also manually set accounts to expire on a specific date (e.g., January 1st).