>We have an application that will be sold to businesses. After a year a given amount will have to be paid by the customers so that they always get the newest version. Up to that point no problem.
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>Here's what I'm wondering about. Some clients are already asking for additions/modifications. How do we manage that?
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>Do we charge them for what we just added? Because there are possibilities that these improvements will be included in a newer version. But to be fair we don't want to charge that customer for things that could be used by others at probably no cost other than the premium they'll have to pay each year. If we charge them do we offer them free services after a new version is released because they're responsible in part for a better version of our software?
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>We also have to keep in consideration that some improvements could become major things added to the software. In the end improvements worth more than the annual amount paid by only one client.
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>What should be the fair way to manage this for everybody (us and the clients)?

Usually we sell the software in the same way.
Fixed price with 1 year free support and updates. After that - monthly support fee, which include free upgrade. For customers which not like to pay monthly support we offer only upgrade.

We do not charge our customers for their suggestions about software improvement. Better software lead to more customers.

But if they like special report or screen which will be not used by other customer and we do not intend to add it in monthly upgrade we advice them to try to produce alone with our Report Generator or charge them.

The line of fair behavior is very slim. But if you will use their suggestion for improvement, it is not fair to charge them.