Aside from multi-classing and taking the Magic Initiate feat I know you are generally limited to the class list. But what about scrolls? It says in the PHB that you can learn spells that are found during your adventures if you spend the time and components (50 gp per level of spell) to learn the spell and copy it into your spellbook. In LMOP you can find a scroll of revivify. If the spell can be written down on a scroll, why wouldn’t a wizard be able to copy that into his spellbook?