Temple of Sun, Temple of Moon, Shrine of Celeste
Somewhere in tanren
to enter ToS you need either an espeon or solrock
to enter ToM you need either an umbreon or lunatone
maybe theres a chimeco sign to remove the entry mons from team?
theres sun/moon related pokemon in the dungeons
bosses of ToS and ToM are a solrock/horde of espeon and a lunatone with a horde of umbreon
the rocks drop lunar/solar shards and you need one of each to enter Shrine of Celeste
theres pokemon in SoC that evolve with sun/moon stones
boss is lunala and solgaleo together
either they drop their heart slates or theres a cosmog egg in the end room
that concludes this idea