I found this article useful when brainstorming about a turn-based tactical combat system, like those found in FFT and Disgaea. Perhaps you'll find some value in it. Some things I want to try with my own system derive right from that article.

Spells can inflict or mend damage on a target; or boost or inhibit a target's ability to inflict or mend damage. Those are the the basics you're referring to I think. grossly reduced.

Spells could also prevent or inhibit a target from moving (grasping vines or a slow spell) or could prevent a target altogether from action (a polymorph-to-sheep) or even do both (turn the target to stone). Spells could temporarily or permanently take control of a target (a mind control spell, maybe a strong negotiator skill) or otherwise turn the target against its own allies (spells named "confusion" often do this). Dust storms could seriously hinder ranged accuracy or flat out prevent them from hitting at all. Spells can make a character act faster or even temporarily freeze the action of all other characters, like a "time stop" spell.

You can have spells that allow your characters to float or fly above the ground, avoiding terrain-based damage or even the reach of melee attacks. If height is a factor on your maps (as it is in FFT and Disgaea) some characters could have a Dragoon-style jump or spider-climb power that allows them to ignore height changes and get anywhere they wish. Spells can teleport your characters, or maybe swap their places with another character on the board, friend or foe.

Characters could change the game map, forming temporary barriers or walls that even last the duration of the battle. Spells could open up chasms to keep melee brutes away or give you a place to cast down your foes...or it can create bridges across chasms to get your melee fighters into the fray. You could allow characters to pick up friends or foes and throw them, either explicitly (as in Disgaea) or as part of attacks, like judo throws or pro-wrestling. (That need not do any damage as it already serves a function of putting space between you and an opponent.) Just allowing a character to shove another character around can achieve this.

That's all I can crank out in like ten minutes haha. I hope that's useful.

What about adding some kind of "bait" spell or item effects?

Like say a mage could plop down a rune that slowly heals anyone on that spot, be they enemy or ally (while damaging undead if healing magic does that). Naturally, enemy units low on health will want to stand on that rune to heal up, but you could place it withing range of all your own guys or have traps in place to get them when they take the bait.

Or have a spell that can create food or drink items in and out of battle (ideally, they stock up on food out of battle and use it for cheap healing or energy in battle). The same items could be dropped (and poisoned) to distract wild animals.

This could potentially be used for "commoner" characters, or just those who aren't supposed to be actual combatants but have at most some buff/debuff abilities to help out with. I wonder if adding some support abilities to noncombatants could make them more interesting to players during things like escort quests or guarding them during defense missions. Like say a merchant being escorted can give you a temporary speed boost, a king boosts your attack during a battle, or some lovable urchin taunts the enemys to lower their accuracy.

How about something like this?

Make it so casting the support spells doesn't terminate the character's turn. For example, a fighter class can cast 'increased damage' sort of spell on himself and attack on the same turn. This would ensure that the player will use the skill because it will always have positive effect on the battle. The catch here is that there should be some sort of limiting factor like resource, e.g. mana, so that player is not constantly buffing himself every turn. Ideally, if balanced well enough, player will have to decide when to use those support skills because that character can only use the skill 2 or 3 times, putting in more depth and strategy in the battle.

How about something like this?

Make it so casting the support spells doesn't terminate the character's turn. For example, a fighter class can cast 'increased damage' sort of spell on himself and attack on the same turn. This would ensure that the player will use the skill because it will always have positive effect on the battle. The catch here is that there should be some sort of limiting factor like resource, e.g. mana, so that player is not constantly buffing himself every turn. Ideally, if balanced well enough, player will have to decide when to use those support skills because that character can only use the skill 2 or 3 times, putting in more depth and strategy in the battle.

I know in Dungeons and Dragons 4th edition, during each turn a player can take a standard, move, and minor action (plus immediate actions for certain spells). Minor actions tend to be pretty weak but are good for minor buffs or utility spells. You could check out 4th edition rules for ideas.

Or, the spells could be 'stored' for later use after casting? Say, it takes 5 seconds or so to cast the spell, but once cast it gets stored where it can be activated later. Each character can only have so many stored spells at a time and casting the spell is generally too long to do during combat. So, before combat each character can prepare some powers to use during a fight and activate them quickly, and in an emergency they can 'recharge' those abilities by preparing them during combat.

Perhaps how the noncombatant characters work? At the start they use up their stored abilities to boost the fighters and after that they default to walking around to avoid combat while casting their utility spells. Characters can walk and cast "preparation" spells at the same time. The noncombatant types stay out of the way but the players can see that they are charging their spells so they are helping out.

Battles with a noncombatant ally thus has the first few rounds where they guy tosses out five of six boosting spells in quick order (perhaps twice in a round because they were prepared beforehand) once he runs out of prepared spells he stayss back and prepares more (each casting takes multiple rounds) and then casts those as soon as possible.

Thus, Utility Preparation abilities are sort of like using items in alot of rpgs, they are decently powerful and take a short amount of time to use, but you have to have them on hand to use. The Preparation abilities however don't cost money and can be restored in short order (so you don't feel the need to horde them). Each character can only have so many of these abilities stored up at a time though and the number of them stored can increase as you level up (or maybe invest in a certain skill tree).

So, most characters will invest a fw points to get Preparation slots that they can use quickly in combat (things like healing, power boosting, or mana recharge being the best). However, more dedicated support guys (like the above noncombatants) invest alot into these preparation slots so they can fill them up outside of combat (or during combat by hiding) and then unleash a lot of them rapidly when needed. I'm immediatly reminded of the Item Caddy trope on TvTropes. Basically a character whos special ability is being able to use and toss out items for the team (read it because of one guest character in Final Fantasy XII has an unlimited number of Hi-Potions he dispenses liberally in combat). With Preparation abilities, the characters would be like that but effectivly have an unlimited number of "items" to use so long as they have time to prepare and slots to use.

Rogues could prepare things like smoke bombs, healers prepare healing salves or spells, fighters use things that boost their combat prowess, etc. The key is getting Preparation Slots, then the abilities to use in them, and setting them up to auto-recharge before combat or prepare them in combat if need be. Using them is quick and doesn't slow down combat.