I'm going to have to agree with Wavelength here.
I'm also going to be a little bit of a jerk here, so... don't come to beat me up just yet. I'm not trying to be a jerk here, but it's going to come off that way.
I could give you ideas all day long about what each class, what it should do, what role it should fill, what skills it should possess... But, ultimately... Anything I would say would be completely without any kind of context. A class system and skill system are meaningless without any kind of insight into how your combat works. What kind of combat are you going for? What do you want the player to be doing in it? What will the enemies be doing and what will they utilize as abilities? Moreover, what kinds of "elements" do you have in your game? By that, I mean things like, do you have just the standard RPG elements of earth, water, fire, wind, etcetera... Or do you have things like Blunt, Piercing, Slashing, Stabbing, etcetera. How does that stuff all interact with each other? Is it just another element to be exploited by the party, or is it something deeper?
Without knowing just what it is you're doing with the rest of your game, it's going to be difficult to suggest any kind of skill as "worthwhile" to your particular system. I mean, you can gain ideas yourself by just playing around in the RPG Maker. Honestly, that's how I've come up with most of mine. I noticed things like, "Hey, I can inflict unique states on my party members or the enemy to do really interesting things other than damage" and made a few like that. I also noticed things like, "Hey, I can select all kinds of different targets, including random targets..." and made some skills like that. I noticed I could have things like "attacks multiple times", and I did a skill or two like that. I noticed I could raise critical hit rates across all party members and played with that. I found a neat little thing in there where I could play with skill formulas so that the more of your HP was missing, the more damage that particular skill would inflict (max HP minus current HP = damage). I found out that I could make my skills run on any stat I liked and have any stat I wanted be the defense stat for the skill. I made skills that increased weaknesses to particular elements. I made skills that made my characters go berserk for several turns and also raise their TP while they were berserk.
But, again, without really knowing all that much about your game... namely all the bits that you slot a "skill" into... I'm not sure I can really help you much at all. If all you want is examples others have used in their games, then that is probably easier. But, some of that might even require explanations on how their battle systems work.
If you want a couple examples of "interplay" from my game, I can give you a few. I have 9 characters, 3 of each "type". I have 3 Speed characters (Agility is their primary stat... it's usually what they use to attack as well as what most of their skills run off of), 3 Magic characters (just like the agility characters, except their builds revolve around the magic attack stat), and 3 physical hitters (one is basically your default tank, one is your basic "jack of all trades" type, and the third is a magical knight that uses skills with elemental effects... except uses the physical attack stat to cast them... unless you level up the skills to use the magic stat instead... but that's neither here nor there). My first character (the one you cannot ever swap out) has a skillset called "Adrenaline". It runs entirely on TP. He has exactly one skill that consumes MP and it's the only healing skill the party ever gets... and it's limited to just healing himself. But, he has the most diverse set of skills, and some of them interplay off of each other, or can be used to set up other characters. He has a full party buff called "Battle Cry" which can be used to buff either Magic stats by 75% or Attack and Speed stats by 50% each. The buff lasts 3 turns, so it's main use is to set up other characters to either use their skills in quick succession, or to inflict more damage with some of their basic skills. I have a zero MP Speed and zero MP Magic skill that are used if you want to just be "quick and dirty" about combat and just want to overwhelm the enemy without having to cast really high level stuff. I mean, they are just straight "User's Agility minus Target's Luck" and "User's Magic minus Target's Magic Defense" affairs, but they get the job done without using MP and they're a good fallback position to have. If you want more interplay between some of my classes, I'll give you my Magic Knight and my Witch. My Witch is the basic "I'm a wizard, I hurl elemental crap around and do damage that way" skillset. The Magic Knight can be a different version of that Witch, or a unique version of a Bruiser type class. Anyway, the interplay. If you level up the Magic Knight's skills along the "Physical" paths, one of the first things you get is the ability to inflict "Weakness to X" at 80%. In and of itself, you can simply land the blow, and then cast the same skill again, and do more damage the second cast. But, wait, there's more. The Witch can cast her spells the turn after him and do more damage that way. So, even if the enemy is strong against physical hits, the Magic Knight can still set up more powerful abilities for the Witch on her next turn. Even more interplay results if a player goes all the way down the "Physical" line of the Magic Knight's skills. At the very last upgrade, he gets a SECOND "weakness to X", except instead of an element, its' an entire damage type (like magic, speed, physical, blunt, piercing, slashing elements). So, instead of only helping out the Witch... He's potentially helping out the entire party, depending on what they've got equipped.
An example of some interplay I have with just a single character is that first one I mentioned who uses Adrenaline. He can eventually learn a skill where, at a cost of HP, he becomes immune to death for a set number of turns. His HP can go to zero, but he will not die, until the state wears off. It costs a massive amount of TP to use it, but it can save your butt in a pinch. Even more, if you've got all the right equipment on you... You can use it in conjunction with one of the examples of skills I used earlier. Yep, you can use it with that skill where it does damage based on how much HP you're missing. Even better, you can level up the Immunity to Death skill to the point where it lasts SIX TURNS instead of the default 3. Even better? It removes 95% of your total HP as part of the cost of those 6 turns. Oh, and higher levels of the other skill that does damage based upon missing health... gets multipliers for damage if you go the right way for a level up. If you do it right, it works like this: "(Max HP minus Current HP) * 3. So, if you cast the "immune to death" skill and it knocks out say... 95 out of the 100 HP you've got... then you use the "does more damage with more HP missing" skill after it, you can do 285 damage with the skill.
I know that's pretty long... I tend to get verbose when I get passionate. So, I'll summarize with this tl;dr version here. The key to simply coming up with skills is just to look at all the options you can have in the skills tab of the database. Likewise, the key to having any kind of "interplay" is to simply come up with skills that are useful on their own... But some other skill can play off of them just as well, or somehow enhance them, and reward the player for smart thinking.
If you need an example of "how to do good interplay", just look no further than Final Fantasy 6. That game is full of that stuff. Especially just in the Magic abilities. I mean, how many players figured out that you could use "Vanish" on an enemy to ensure a 100% hit rate for any magic spell... and then cast either "Doom" or "X-Zone" on the same enemy for an instant win without the internet? How many people figured out that you could mix the Genji Glove (equip two weapons) and the ability of "Sword Tech" to get 8 hits each cast? How many people figured out you could equip the item that let you cast two spells back to back... and then have a special character mimic those double casts for FOUR CASTS of the same spell... two of them free?
Just gotta look for ways to have interesting skills that fit your game and your battle system... such that they work on their own. Then, come up with other skills that also work on their own, that can play off of those other skills if a player figures it out. You know, like dousing an enemy in oil (which might normally reduce the enemy's speed... like say cast Slow on them... or inflict some other state on them that makes it difficult for them to fight)... and then casting a fire spell on them, which might do more damage if they have that state on them (simple to do, even without a script. You can set it so that Oil imparts a 100% weakness to Fire... then you just make sure that all fire spells come with a "remove Oil state" attached to them).
Okay, I'm done. Again, sorry that's so long.
