I read (or actually reread) Ben Zion Wacholder's Messianism and Mishnah. Wacholder summarizes his argument on pages 34-35:
"As I see them, the mishnaic allusions to priestly and levitical privileges, the sacrificial cult in the Temple, as well as the rules of purity and pollution, refer, unless otherwise indicated, to Halakhah of the First Temple which will be reinstated in the Third Temple, but which does not necessarily describe practice in the Second. Rabbi Judah Hanasi transmits how the Halakhah governs Jewish life at its optimal, not in a period in which institutions such as the jubilee, the Hebrew slave, the nazirite, priestly and walled cities, are in abeyance. Moreover, a situation in which certain commandments had fallen into desuetude may have suggested to the early rabbis that the observance of other commandments was halakhically imperfect as well...The redactors of the Mishnah presume a time when the Land of Israel extends from the Euphrates to the 'River of Egypt,' when all twelve tribes are settled in the Holy Land, when the Davidic dynasty reigns in Jerusalem, when the priests are of unblemished pedigree and observe the rules of purity to the highest degree."
Wacholder's point is that many of the Mishnah's laws were intended to be for the Messianic Period, since they cover many topics that were irrelevant to the time of the Second Temple. Elsewhere in this booklet, Wacholder talks about rabbinic debates over whether the Second Temple was even sacred---since it lacked the Ark of the Covenant, the fire, and the Urim and Thummim---or if it were merely constructed for symbolic purposes, i.e., remembrance of the First Temple (pages 29-30).
For Wacholder, this insight accounts for certain details of the Mishnah, such as the similarity of the Temple in the Mishnah to the Temple of Ezekiel 40-48, especially in terms of dimensions (cp. Mishnah Middot 2:1 and Ezekiel 42:15-20), rather than the Second Temple, or the Temple of Herod (which was an expansion of the Second Temple). For Wacholder, the rabbis experienced the past and gained a foretaste of the World to Come when they talked about halakhah, even if the halakhah they were discussing was not always applicable to their day and age.
A lot of this makes sense to me. But I was confused by some things that Wacholder was saying. For example, on page 22, he talks about a rule in Avodah Zarah that states that Jews shall have no social or economic interaction with a pagan three days before an idolatrous feast. Wacholder believes that this was a rule for the time of the rabbis, but also that it was saying how things were in the days of King David, and how they would be again under the Messiah. But does that mean that idolatry will be tolerated in the times of the Messiah---that people will be engaging in idolatrous feasts?
I think that the rabbis believed that Jews were required, or encouraged, to observe certain halakhot, even though the observance would be imperfect. For example, I remember reading in Leviticus Rabbah that a Jew who keeps the Sabbatical year is a gibbor, a mighty man---especially since allowing his land to lie fallow would be difficult in a time period that had heavy Roman taxes!