For the most part, RPGs stick to a certain kind of battle system adamantly. I pretty much divide these up into 3 separate types: Action, Regular, and Tactical. Regular battle systems are what RPG players are probably most familiar with--the menu-surfing standard made almost synonymous with the genre by the infamous Final Fantasy and Dragon Quest series. Action RPGs are practically the exact opposite, having you actually PLAY the game rather than select commands and hit a Confirm button thousands of times. Although not nearly as common as Regular battle systems, Action battle systems are just about as old to RPGs, with games like Crystalis and The Legend of Zelda series in their early history, and only seem to be getting more common. And then there's the Tactical genre, which has you moving units across maps in predetermined distances of movement, where distances, heights, terrain, and ability side-effects are often the true deciding factors in combat. It's hard to explain, but chances are you've encountered more than one before, and if not, well, I dunno, go on Youtube and find a video of someone playing Final Fantasy Tactics or a Fire Emblem. You'll get the idea pretty quick.
Anyway, RPGs pretty solidly stick to one of these genres, in most cases. An action RPG's gameplay is pretty much never gonna deviate from that battle system, save maybe for the occasional annoying minigame here and there. Outside of of wandering around dungeons, towns, and the map, and playing the previously mentioned annoying minigames, you're not going to be getting much real gameplay in a standard RPG, just hundreds of interruptions where you "battle" enemies with menu choices. There's really not a lot of attempts in the RPG world at mixing up the systems themselves, only at adding features and innovations to the existing ones, which often just ends up making them needlessly complicated (the actual, real world is not as complex in its laws and natural balances as Star Ocean 2 and 3's battle systems are) and/or silly (Final Fantasy 12's requiring experience to equip a better suit of armor than they currently have clearance for, even though the skill to put it on would have already had to have been mastered for them to wear the currently-equipped clothing).
However, there IS a certain innovation that I'm fond of that happens on very rare occasions: the use of TWO (or even all three) of the battle systems in a single game. No, I'm not talking about just the fact that you can (and really do have to quite often) open up a menu during an Action battle to give commands, like in the Tales of series or something. I mean real, distinctly different battle systems being used at different times.
The earliest game I know of that did this was the good old NES title, The Magic of Scheherazade. Now, I admit, I am very, very fond of this old treasure, and nostalgia plays a great part in that--I consciously know it's only a fairly good RPG, but darned if I don't just love it to pieces. TMS was pretty unique, though, in that it employed both an Action battle system, AND a regular Menu one. You travel from one screen area to another (just like The Legend of Zelda 1), and on each screen you can encounter enemies that you have to fight off by actively controlling your character and having him move and attack. But, each time you move from one screen to another, there's a chance of you getting attacked, Random Encounter-style, and having to fight an enemy party in a standard (if extremely simple) menu battle. It's neat, the way it's separated, and it breaks up the gameplay a bit so it's not all just the same thing over and over again.
Of course, the most famous example of this combination of RPG genres would probably be the Suikoden series (the numbered ones, at least; Suikoden Tactics predictably only features Tactical combat). Suikoden games primarily employ your standard, menu-based battle system for most of their battles; however, in each game there are many times when you have to win a round of Strategy combat. Though the tactical battles are usually not as complex or developed as the regular ones are (the first Suikoden's strategical warfare is only a few steps up from Rock Paper Scissors), it's a great break that works very closely with the game's plot to help you get drawn into the game's events. In addition, the series has one-on-one battles that actually qualify as an entirely different genre of RPG battling altogether, in my opinion, which further give the player some plot-related variety. It's done very well.
The last game that I can think of immediately that has a combination of gameplay types would be Bahamut Lagoon, for the SNES. I'd also consider it the best example. Battle is a perfect mix between Strategy and Menu battle systems--you move your units across the field and use abilities and terrain just as you would in a regular strategy game, but you can also have a unit personally engage an enemy instead of just attacking on the field. During this personal fight, you get a turn of regular menu combat with your assembled troops. It's all easy, it's all smooth, and it's all connected, yet separated very distinctly. Quite different and innovative.
Of the 3 games/series where I've seen it implemented, all have done it in a way that was overall positive and refreshing. So, really, why don't more companies do it?