I think that wizards should be able to switch out their cantrips in a similar manner to how they prepare spells - ie you 'prepare' a limited number of cantrips and then cast them at will, and the cantrips that you start with are the ones written into your spellbook initially.
So let's say a wizard finds a spell scroll for a cantrip from the wizard list that he doesn't currently know. The DMG says this type of magic item has a rarity of "common", meaning that scrolls like this should be inexpensive and easy to find, even in (magic-item-sparse) 5E. Can the wizard learn a new cantrip from scroll?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.