It depends on what interests you. But I'd say some common things that all types of pilots would benefit from are:
* payware scenery for your favorite airports
* better ground textures (e.g., Ground Environment X Europe/North America)
* better weather engine and environmental textures (Active Sky Evolution and Real Environment X)
Finally, if you haven't yet flown online, this adds a whole new level of realism, especially flying in areas where Air Traffic Control is active. So check out and join AVSIM and IVAO.
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