Well, if you think about it, Lina never uses that much variety of spells. If she's a 20th level sorceress, she could easily have access to all of those spells. Plus, at 20th level, she's got a +10 base attack bonus, and though she's no Gourry, she's still better than the majority of the world.
DragonSlave is a 9th level spell, basically a fireball that does 1d6 damage per level, with a radius of 15 ft/level. It has a casting time of three full rounds.
The Ragnablade (I think that's what it was called) is a 9th level spell that creates a gargantuan keen vorpal greatsword +5, which the caster can wield as if it were a shortsword. The wielder immediately gains full proficiency in this blade, and can use it to attack all targets in a 5-foot wide swath, out to a maximum range of 20 feet. The blade acts as a touch attack and creates a pocket antimagic field that allows it to penetrate even magic shields, so basically only Dexterity helps avoid the attack. The blade deals 4d6+5 points of damage, and this damage cannot be healed in any way for at least one day. Oh, and the blade also ignores hardness.
The Gigaslave, though. . . . That's a little tougher. I'm working on a mechanic for that, but I don't plan to release it yet.
The question is, though, would you want to play in this world, or would you just want to play Lina and co? I ran a fun game on New Year's Eve two years ago, where we were all Slayers folks. We had Lina, Zel, Gourry, Naga, and Sylphiel work together to save the world. The plot was that Naga found a magic item that let her travel back in time ten years to when a great sorceress who could grant wishes was still alive. Then, in the past, she wished that "the small-breasted sorcerer girl who I never liked" would lost all of her magic powers.
Now, we were working under the hints that apparently Naga and Amelia are related (similar hair, and Amelia has very large breasts for her young age, plus she once mentions having a very tall sister). So since Naga made her wish ten years in the past, the wish took effect as if the Naga of that time (who had not yet met Lina) had made the wish, instead of the Naga from the future (who had met Lina). Thus, the small-breasted sorcerer girl who the Naga from the past never liked was her younger sister Amelia. And since Amelia lost all of her powers, the group proved unable to defeat Shabranigdo at the end of season one, causing the world to be destroyed.
Naga travels back to her own time, and gloats at Lina about what she has done, when suddenly Amelia pops out of existence, and the rest of the world begins to disintegrate. Everyone pounces onto Naga and forces her to take them back in time to set things right, but Naga gets confused, and they ended up jumping back several millennia to the war between the Mazoku and the Dragons. They got to fight against Xelloss (who stole the time travel necklace from them, plucking it off of Naga's cleavage), and all manners of demons and dragons before they managed to recover the magic item. Then they time traveled again, and managed to head off Naga from the slightly-in-the-past present before she could make her wish.
It was confusing, zany, and fun, the way Slayers was meant to be.
