It may happen someday. For example, the Ancient Cave hack for Final Fantasy IV. Similar to a side quest in the game Lufia II: Rise of the Sinistrals, the patch implements the feature of the mentioned game implemented to Final Fantasy IV. If that patch was made for that game, it would work for the USA Rom of Final Fantasy II on the SNES and the Playable Golbez Edition patch by fedorajoe.
Unlike the Final Fantasy V: The Ancient Cave romhack where they have to lose everything at level 1, their armor, skill, magic, and summoned monsters they learned in the main game is kept intact including items you've had in the main game. In other words, you don't lose everything other than level stats. Their MP is kept intact but whatever MP they have does not increase until they reach that level and they don't gain MP if it's already at 999. Even though you have to reset at Level 1, you don't lose a lot of stuff.
This is to not only balance out the game, but to prepare for the most difficult fights ahead.
We have EVEN MORE themes now, but some old ones are gone...