Since P7zip has been updated to 9.13, keka should be now able to compress/decompress using LZMA/LZMA2 and also PPMd; it would be nice to be able to choose the 7z algo to be used (by default and/or by archive).
You mean an option to select LZMA/LZMA2 or PPMd? Is in the TODO list (if not, I miss to write it) but also I have in mind to choose it automatically depending on the type of file to compress (PPMD for doc, txt, text based, for example).
Just be careful: many users have no idea about the algorithms, a sort of automatic (intelligent?) selection should be always offered.