There is an idea quoted in the name of the Ar”i (it was actually first written already generations after him) that in the future (perhaps it was olam haba) halacha will follow Bet Shammai. Regardless of who actually said it first, does this have a source in Hazal?
None. And no-one knows who dreamed up this dubious notion. Nor does it matter.
The הלכה will be determined as delineated by רמב”ם in his הקדמה למשנה תורה: by studying, in depth and after much experience, all relevant Hazalic materials and the commentaries of חכמי ישראל לדורותיהם. This is how all the ראשונים ז”ל learned and taught תורה and הלכה: רש”י, בעלי התוספות, חכמי ספרד, חכמי איטליה, חכמי פרובנס – everyone.
This will be the case until חידוש הסמיכה, which will lead to establishing a real בית דין, which in the fullness of time will lead, by way of a measured and evolutionary process, to a renaissance of תורה שבעל פה.
אם תרצו – אין זו אגדה.