Hi Dark Dimension, thanks for your answer.
The problem with the "different price" option is that, and correct me if I am wrong (I am new to itch.io), it would have the same price regardless of you having already bought the base file or not.
With the pateron solution, the problem is that you have to get people to go to your patreon, and create an account if they don't have already (this is my case). If options are part of the itch.io store, you will definitely impact more the people who are already there.
My real need is a per product subscription (like for Resharper for example for those familiar with it): users will get updates for free for a year, and then have to pay again if they want to keep getting the newer versions. The other ideas were suggestions based on what I have seen people here and there ask for, so I thought since these needs are kind of related, let's put them all in the same post.