If you are going to make a cross-platform library, usually one of the best things to have is prior experience on all of the platforms. Do you have that? It is possible to venture out into the unknown, but generally it means you will make bad decisions and the library will reflect it.

You also mention making a Unity plugin. Again, things are easiest if you have prior experience doing thing.

As for listing pros and cons, well, it depends on your implementation choices.

Some things must be specific to each platform. Some things have the potential to be merged. Modern games are exercises in software architecture, and there are thousands of decisions to be made. You can decide to have a separate implementation for each platform, and with the right architecture this is the correct choice. Or you can decide to have a single monolithic implementation, and with the right architecture this can also be the right choice.

Right now Google can find thousands of libraries designed for cross-platform mobile development. Even so, a very small number of high-quality libraries have become popular. Perhaps you could study out each one and learn (on their websites and forums) why each of them became popular. Each one has made different design choices, so I suggest you find out why the 0.1% of them succeed and why the 99.9% of them fall by the wayside, and learn from their mistakes.