I'm playing D&D 5e. PHB (Chapter 10: Spellcasting) states that (emphasis mine):
Regardless of how many spells a caster knows or prepares, he or she can cast only a limited number of spells before resting. Manipulating the fabric of magic and channeling its energy into even a simple spell is physically and mentally taxing, and higher-level spells are even more so. Thus, each spellcasting class's description (except that of the warlock) includes a table showing how many spell slots of each spell level a character can use at each character level.
I want to emphasize this aspect in my homebrew world so I have come up with the following house rule:
Inner Reserves: A spellcaster can exceed the number of spell slots normally allocated to her at the expense of getting fatigued.
When a spellcaster runs out of spell slots she can access her inner reserves and push her physical and mental abilities to their limits. Doing so, allows her to cast a spell but gain an exhaustion level equal to the level of the spell she cast. For example, if the spellcaster casts a Level-2 spell she will gain two levels of exhaustion.
The exhaustion takes effect at the end of the spellcaster's turn.
I would like to use this house rule for several reasons:
- Fair Exchange: In my eyes, this is a fair exchange. Gaining one level of exhaustion will most likely not make a difference in a battle but that is also the case for a Level-1 spell. On the contrary, when gaining two levels of exhaustion or more, the PC becomes extremely vulnerable. So this is a high-risk high-reward scenario. If you are about to exhaust yourself you better make sure that you will win the fight.
- Dramatic Effect: I think it goes without saying that such a mechanism can lead to some very dramatic moments. I can imagine several scenarios where a spellcaster exhausts herself to heal the group or cast one last fireball that ultimately saves the day.
An immediate problem I identify is that this rule allows all classes to cast 5 more Level-1 spells (or less at higher levels). This may break some classes that are designed to cast very few spells per day (e.g., Warlock, Paladin). On the other hand, I feel that this imbalance is mitigated by the severe consequences of high exhaustion levels. I.e., I don't think that it's viable to abuse this mechanism.
- Will this house rule completely break the game?
- Which classes are going to be affected the most?
- Are there any other unforeseen consequences introduced by this rule? (i.e., unrelated to classes)
Finally, if this rule does not completely break the game, I would ideally like to read about how you'd improve it, but I feel that this may push
rpg.stackexhange.com's rules a bit, so feel free to omit this part.
EDIT: @encryptor's answer includes some excellent points. I have edited the rule's description to address some of them.