I will add a caveat to posts made by
@Milennin and
@velan235, that whatever skills you add to your game need to support your game's core mechanic, or core challenge.
You're making a game on RPG Maker, which means that your core mechanic/challenge is for the player to deplete their enemy's HP to zero before the enemy depletes the player's to zero. A player's skill is determined by the rate at which they can deplete the enemy's HP; the faster HP disappears, the more skilled the player is. In most RPGs, DPS is a direct measure of the player's skill. In most RPGs, incoming DPS is a direct measure of difficulty. Another measure of difficulty is how the enemy can adjust incoming DPS and survive longer. Enemies that can hit hard and keep the player from hitting them hard are even harder.
Now, let's put another layer around this mechanic. The skills you add to the game should really just affect the rate at which damage is dealt or received. Buffs and elemental weakness exploitation are pretty classical means of adjusting this rate.
All things added to gameplay should have as few degrees of separation from "DPS or adjusting DPS" as possible. Because of this, skills that don't do damage such as "Defend/Guard/Wait" are hardly used. So, I suggest that all skills should do damage, even skills designed to adjust the rate of damage per turn.
An enemy buffing itself to slow the rate at which the player depletes the enemy's HP should serve as a little side challenge that briefly diverts the player's attention from the core challenge. After the buff is defeated, the player should be routed back into the core mechanic/challenge of depleting HP. To defeat this side-challenge, the player should have a reliable toolset to counter these enemy buffs. The ability of an enemy to counter the player's buffs can add more difficulty to the enemy, particularly if they are smart enough to use it.
This is all a description of function. Designing things to be purely functional is kind of boring. The other side of this coin is flavor. In addition to the skills you design being functional, they need to be flavorful. They need to be a part of the world and be an extension of the character using that skill. Skills should be as much a gameplay mechanic as it is a part of the story. Flavor should be second to function, but flavor is just as necessary.
But above form and function is a trait that all skills must have: the player must see them as reliable. If a skill doesn't do damage or reliably adjust DPS (read: add a status effect), then that skill is worthless. If the player's first use of a sleep spell doesn't put the enemy they choose to use it on, even a boss, they will never use that sleep spell again. So, in addition to having high-but-not-too-high success rates, things like flavor text and even the enemy's graphics need to convey to the player what skills might be useful. And a precedent should be set early that the skills the players learn will be reliable and useful going forward.
In my opinion, healing is something else outside the typical "DPS or adjusting DPS" mechanics. Depending on how you design your game, you might not even need healing mechanics, and usually stem from enemies having gigantic HP pools but doing relatively little damage to the player, while players have tiny HP pools but do immense damage. Some could argue that such mechanics are imbalanced and require a healing option (for the player of course) to keep balanced, but those ratios are also the norm in JRPGs (the trend-setters
Dragon Quest and especially
Final Fantasy in particular). But if enemy and player have comparable HP/DPS ratios, you might not even need to bother with healing mechanics during combat, and could completely restore HP after combat ends. I'd consider healing to be a tertiary component to be planned and implemented after the "DPS and adjusting DPS" core mechanics.
Stealth and tanking are just mechanics that can adjust outgoing and incoming DPS based on implementation; in fact, stealth and tanking are flavor rather than function when you really think about it.
Stealing mechanics distract the player from core gameplay if it doesn't do damage, and is particularly useless if the player can't rely on it to work at a reasonable rate. In my experience, stealing doesn't see a lot of use.
Summoning is interesting in that they're really just expensive and flashy spells in some games, or means of controlling enemy behavior or crowd control in others.
After the core "DPS and adjsuting DPS" paradigm and the tertiary healing mechanics are implemented, stealth, tanking, stealing, summoning, and any other flavor-over-function mechanics should be designed and implemented last and should be useful and reliable and not distract too long from just doing HP damage.
But this is my personal design philosophy. Take it with a grain of salt.