I'm going to mention a lot of turn-based strategy/ Japanese developed games here, but I encourage the community to find other series that need much revitalizing. So ahem... lets get started.
1. Final Fantasy Tactics 2 - This is definitely one of those dream games that will sadly never see the light of day. If it were to ever happen it would have to be written by Yasumi Matsuno (that in itself is definitely not a huge possibility, as i've heard he has given up his "dark" style of writing) with music by Hitoshi Sakimoto and Masaharu Iwata. As far as ideas for the game, it could be a prequel to FFT set during the fifty years war or maybe after, showing what happens after Delita takes over.
2. Legend of Dragoon 2 - The original was developed by Sony I believe and its a really awesome game. I've yet to see a game with such a cool battle system (The timing mechanics[Though something similar did occur in Lost Odyssey, showing how invigorating its appearance can be]). A sequel or even prequel with modern graphics, a revamped leveling system (from what I remember only bosses could really level you up) and a better story could make this one of the best RPG's.
3. Ogre Battle Saga Episode IV (Or Whatever) - Another Matsuno written game (can you tell i'm a fan?) and a gem of a series, that has recieved very poor treatment for the quality of the games. Although a dream, another ogre battle game akin to the N64 one (person of lordly caliber) would be amazing.
4. Super Mario RPG 2 - Though technically paper mario is a spiritual successor, the original lacks a true sequel. Admittedly I havent played all the way through the original game, it still has an endearing gameplay.
5. Secret of Mana 2 - Rerelease the original on PS3 and Xbox 360 with HD support, local or online coop gameplay, and then develop another downloadable sequel. Could net a lot of money.
6. Xenogears 2 / Xenosaga 4(?) - Although a bit narrative heavy, both Xenosaga (though a bit convoluted and pretentious) and Xenogears were highly enjoyable RPG's. A narrative driven game is just what the industry needs, instead of curse-laden, bullet spraying, mindless FPS games. (Don't rant, I like FPS too lol)
While i'm sure there are many, many more worthy RPG's that deserve sequels (Chrono Trigger!!!) I'd like to discuss one more thing. Imagine how much duller these games would be with sequels. Would a sequel lessen the magic of the original, or enhance it? Its kind of the same with movies. Everyone that has seen the Disney movie "The Fox and the Hound" knows that its a pretty tragic, yet heartwarming story. Have you seen its sequel though? I sure as hell haven't. Why not? Well, though it may seem ironic some things just don't need sequels. As they say lightning doesn't strike twice. Just something to keep in mind.
So what do you think? What are some RPG's that NEED to be made? I've named many Square/ Japanese RPG's. Whats some western RPG sequels that need to be made and why?