|Sorry man, this is all I memorized today!|
Granted, a player might find a useful reason in the dungeon to need a Floating Disc. In this case, he could go back to camp, sleep, and come back the next day with just that spell. But by then the opportunity may be gone. And it's not all that exciting and spontaneous.
So I was trying to construct a house rule which would allow players access to all spells (assuming they are high enough level to cast them), but at the price of having a chance of failure. That way, when that odd opportunity comes up to cast Ventriloquism you can dramatically try to wrest it from your memory and cast it at just the right time.
So here is the rule. As usual, feel free to suggest areas for improvement.
Magic Users and Elves no longer have to select their spells at the beginning of each day and no longer require spell books. Instead, they have access to all level-allowable MU spells from the list of spells. However, any time they wish to cast a spell, they must roll at or under their INT score on a d20. If they succeed, they can cast the spell. If they fail, they cannot pull the spell from their memory and that particular spell can’t be cast that day. In such a case, the MU/Elf can’t take any other action that round. However, they can still attempt a different spell in a later round or encounter – just not the same spell that has already failed in any given day. If a spell is successfully cast once in a day, it can be cast again (assuming a spell slot is available) without rolling. In no cases can an Elf or MU successfully cast more spells by level in a given day than the number allotted in the “Spells by Level” tables. Thus, a 3rd level MU (Conjuror) could cast no more than two 1st level spells and one 2nd level spell per day.