Gonna talk about egg appearances and symbolism
>Temmie brings an egg to class almost at the very start of the game, in the hope it will help her complete her project.
>Forgotten Man eggs are "not too important, not too unimportant" yet are the only Dark World objects except the Black Shard which are unchanged between Dark and Light Worlds.
>Forgotten Man eggs count as "issues" in the PlayStation achievements, which probably refers to the phrase "without issue" applied to people not having children/heirs.
>Pipis appear to at least some times be eggs of Spamton, or at other times act like rapidly-multiplying invasive clams. Either way, they are living things encased in a shell, which can be consumed.
>Forgotten Man eggs also seem to be able to multiply on their own a little bit, given how they function in Asgore's fridge and Sans' egg basket.
>Both types of eggs showed up in Noelle's copy of Cat Petterz, and seemed to act like living pets rather than pet eggs (albeit weird ones); neither were able to hatch, but the pipis replicated on their own while the Forgotten Man egg accepted treatment like a pet before leaving "due to happiness".
>Susie is known to have searched "can harboil eggs hatch" on a school computer, seemingly in reference to Temmie's.

Basically, eggs seem to keep cropping up as something acting not as it seems, and they seem to act more like little creatures that just resemble eggs rather than normal eggs. I don't know what this means except that it is a suspiciously recurring motif, with like three independent samples (Forgotten Man eggs, Temmie's eggs, and Spamton's pipis)