My personal favorite (I made up myself): The Gematria of Challah (43) is equal to the number of times the Kohen Gadol sprinkles blood on Yom Kippur.
Cute Drashos from the Gematria of “Challah” bother me, though. What we take off isn’t called “Challah” in Chumash. That’s a Rabbinic term, presumably just borrowed from colloquial usage (though that is a Machlokes Achronim, I believe between the Nodah Bihudah and Abudraham, among others). So I’d find it strange for the “meaning” of the Mitzvah to come from something that the Mitzvah would later me called.
None of this should apply to Challas Chutz La’aretz anyway (certainly nowadays), which is just so we remember that the concept of Challah exists.