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Any formats that are well documented and for which there is open source software that can read it.
The problem with formats that are 20 years old today is that many of them were proprietary formats that only one program could read, and which required difficult reverse engineering to convert. There are converters for almost all formats that have ever existed, as long as SOMEBODY knew how to convert them.
I strongly suspect that EPUB, PDF, and even MOBI will be well supported with some sort of open source tools 20 years from now, even if only as a one-way conversion.
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