Let's say there is no mana/MP. Healing items are relatively sparse. There are two types of skills -- I'll call them "skills" and "spells" for the sake of argument. Spells are often more destructive than skills, but they have the side-effect of causing a particular variable, which I'll call the Field, to increase by a given amount. Once the Field breaches a certain limit, adverse status effects start accruing on the player's party (and some monsters). If it hits a maximum, a certain amount will be discharged from the Field and a single round of heavy damage will be dealt to the party. Let's ignore any story-related problems and look specifically at the gameplay aspect.

It's easy to set up a lot of variety within this framework. Simpler options include items or statuses which deflect or partially deflect the field discharge, and then deliberately triggering it. Skills could be used to discharge a portion of the Field and damage the enemy while doing so.

The main purpose here is to have a new way of constraining the way spells are used.

This is a pretty brief sketch, but I wanted to present it here to see if it's already been used somewhere or if there are ways it could be improved. I'm planning to implement it in some form in my current project.

EDIT - I notice I've posted this to the wrong forum, if a mod could move it I'd appreciate it...

I think you posted this in the right section :-) I staff the gameplay section but I'm not a mod, so it's really up them though.

I don't think I understand your system. Can you give specific examples? Like if you use fire three times consecutively then the field discharges fire damage to the party? Is that what you're going for?

In Chrono Cross for PS1, battles have a field that changes based on the elements of magic used. For example, if anyone (enemy or party member) uses fire three times, then the field causes fire spells to be stronger and ice spells to be weaker until someone uses a different element (I think - it's been a while since I played it). I think some skills even require that the whole field be a specific element before you can use them.