As long as there are popular video game series, there will be video game spinoffs. While some of these end up being great, like Portal or the Mario Kart games, it's much more common for them to fall completely flat. We've played some pretty bad spinoffs over the years, and here are the worst of them.
If you want to ruin any good fighting game, just make it into any other genre of video game (with the obvious exception of Dead or Alive Xtreme Beach Volleyball). SoulCalibur Legends was honestly just uninteresting. The controls were crappy, the gameplay was repetitive, and the inclusion of Lloyd from Tales of Symphonia did not make up for the fact that NAMCO Bandai refused to release so many of the Tales games in North America. In the end, most fighting games should stay fighting games.
You can take any non-fighting game franchise and ruin it just as easily by trying to make a fighting game out of it. Remember the horrible Double Dragon fighting game for the SNES? Or what about Star Wars: Masters of Teräs Käsi. However, the most recent—and in my opinion worst—abuser was Castlevania Judgement. The game was blatantly unbalanced, most of the characters weren't even recognizable by hardcore Castlevania fans, and, for the love of God, fighting games just don't work with Wii waggle controls.
I shouldn't even have to tell you why a console-based RTS is a bad idea. A controller simply doesn't give you the amount of accuracy and precision that a mouse and keyboard do. However, Halo Wars took one of the most beloved FPS franchises of all time and shoehorned it into a game that was barely playable. It's a good thing that Halo: ODST and Halo: Reach came out so quickly after Halo Wars, or this game could have left a bad taste in our mouths for the entire duration of the franchise.
Pac-Man 2 was just awful. What idiot thought it would be a good idea to make a point-and-click Pac-Man adventure game controlled with a light gun? Really? Pac-Man is a very simple formula. Yellow circle eats smaller circles and avoids ghosts. That's it. How on earth can you screw that up?
Street Fighter 2010 was an NES game that followed the exploits of Ken, only it took place in the future where he was a cyborg scientist defending the world from an alien invasion. What part of this sounds remotely like a good video game concept? The controls were horrible, the difficulty was through the roof, and frankly, it just wasn't Street Fighter. This game failed from square one.