I have a very vague timeline and idea for the gameplay, but since you're the writer I'll leave the details to you. Wizards are semi-rare in this world, there's a college you can go to in order to learn basic spells but only people born as a mage can learn advanced spells. Each game has a mage that joins your party. Each game (except 4) is set in a region.
Fantasy Worlds - Year 1000, all medieval and the like. Evil King I (also an evil wizard), is killed at the end and overthrown by the leader of rebel people, I'll leave the details and character development to you.
Fantasy Worlds II - Year 1500, kinda steampunky (they have engines and muskets and stuff, there's a very basic robot boss), Evil King's wizard descendant (they're all wizards) is trying to regain control by leading an army of evil monsters summoned from hell place (you name it because I suck at naming, hence "Evil King" and "Rebel people".) and you have to overthrow him (again, you fill in the blanks).
Fantasy Worlds GX - Year 1530, there's robots and mechs and it's all very steampunky, here's your original plan for GX which you can change to fit the new storyline:
Set about 30 years before FW2, Generation X takes place in a region called Kloud just east of the Ethren region from FW2, known for it's peaceful community. When the small town of Scyzer is bombed by Battle Ships from the East of Nocturne, it awakens the shadow beasts from Hibernation, along with their prince, Pluto. The shadow beasts wish to finally destroy their sworn enemy, Crystallia (otherwise known as the Enigma), but Pluto refuses, resulting in the shadow beasts overpowering him and Pluto, along with all of Kloud, become enemies of the shadow beasts.
The main character, Breeda, along with his best friends, Reed and Leela, agree to help Pluto by venturing across Kloud, and it's surrounding islands with fellow friends met along the way to enter the Sviesa (light) Temples to summon the power from the Giganti Visagalis (Almighty Giants) into the Demonatta Blade, the only weapon known that is strong enough to defeat the Shadow Monsters. They later discover the true reason for the bombing, and the culprit, and soon learn it is all leading up to something much bigger...
Fantasy Worlds III - Year 2000 - All futuristic and shit, a Final Fantasy 7-style setting. Yeah, not much like the original FW3... This one I'm not so sure about on plot details, but I'm thinking a similar story to FW1 in a modern setting.
Fantasy Worlds IV - Year 2020 - The final game in the series. Setting is pretty similar to FW3, but with TIME TRAAAVEL (woo). You get to travel between the worlds of all previous FW games (with the exception of 3, as it's pretty much he same area) and with two future time periods, one the end of the universe and one in space after the main planet it's set on explodes. Big epic ending. I'm thinking maybe have little hints through the old games that will make sense after completing 4 (an example would be some kind of unexplained scraps of metal in a forest in the original, that are actually parts of a crashed spaceship that was time-warped into the past, or some characters referencing things through the games that are actually from FW4.)
Everybody get up it's time to slam now
We got a real jam goin' down
Welcome to the Space Jam
Here's your chance do your dance at the Space Jam
So, I decided I'd reboot FW, because the old games are literally just reskins of 08jackt's games.