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There was an AMA or interview or something where the DnD team said that they didn't develop spells based on the number of resistances or immunities, it is more to do with theme than anything else. Even before I heard this, I was always of the opinion that spells should be reflavoured to fit a theme, a dragon sorcerer focused on acid should be able to choose an acid version of fireball, it shouldn't cause any real problems.Be careful giving away reflavoring spells. Damage is based on types of damage and ease of countering or having monsters that are less affected by it. Fire is the most common type of spell damage, but there are many monsters that have resistance to it and it is also the first type other people would take to be resistant to. Look at the cantrips and how they do different amounts of damage and add other minor powers like slow, or burst or something else. You would be giving casters a big boost in power if you allow them to reskin spells like fireball to acidball or even the cantrip fire bolt.
I may be more open to creating spells that so something from scratch or even allowing some of the inexperienced player to reskin if they took downtime and did research spending gold.