You have a project which has been developed 5-6 years and now you want to build a new project which will take only 1 year. There is obvious discrepancy leading to only single question: Do you have Product owner who understand current system, understand needs for a new system and is empowered by management to define priorities for features implemented during next year? If the answer is no then you have a big problem and until you solve it you don't have to continue. You need such person who will have responsibility and power to define the project based on business / user / customer real needs.
You have 25 features but what is size of your features? The most common description of the feature is user story. User story is not a documentation of the feature - it is "promise" of future discussion / communication which defines the content and constraints of the discussion. User stories should be small - no user story can span more then one iteration (sprint). If you have larger features you can call them Epics and Themes. Theme is whole area / business domain and it can be divided into multiple Epics. Epic is large set of features defining some main functionality and it can be divided into multiple user stories.
User stories, Epics and Themes defines product backlog but only User stories should be planned for an iteration. Once something is defined as Epic or Theme it means that it is too big and uncertain for a team to make a valid estimation and it must be divided before it can be planned. It's product owner's job to define user stories and to divide epics and themes.
It is very common (and valid) that not all features are known at the beginning and not all of them can be defined at the beginning of the project (if you try to do it you are going back to waterfall). Product backlog in agile is defined and modified continuously during whole project development. If you need a spec / scope of your project let's try to define themes and possibly the most important epics. This scope will not be fixed. As you build an application you can find that some theme / epic / user story is obsolete or your schedule will not allow you including it to a release. In the same time another more important theme / epic or simple user story can be added.