Well, it's always been known that crossovers only usually happen when a company or product is selling badly. An example of this happened a few years ago when the Rugrats met The Wild Thornberrys. A few months later, their showing were rarer and they stopped making more, or so i've heard.
Also, plumber meets blue talking hedgehog? I dunno. I like the idea, but I still say it's cheesey. It would be good in a Super Smash Bros., or in a sports game, but it could be good in any, if done well.
The difference here is that these aren't characters that have absolutely nothing to do with one another (well, yes they are but hear me out). Sonic and Mario have been the subject of videogame debates for over a decade and a half. Being the mascots of two of the biggest gaming giants in the world at the time, practically everybody who had played games from both series had eventually wondered at one time or another, "I wonder what would happen if these two fought/teamed up?" It's been a crossover that's been waited for by a large number of people for many years. Heck, certain gaming magazines wrote articles and made cover-art based on the premise. Other crossovers, like Scooby Doo meets the Three Stooges or, as you mentioned, "The Rugrats Go Wild", are basically just cash-cows attempting to bring together two series that nobody would
ever expect to come together (unless you're a huge Nicktoons junkie). Anyway, bottom line is that there's a big difference between trying to breathe life into two dying franchises by mashing them together and bringing two legends into the arena for a fated duel that everybody hoped would come someday.
For along time I thought that the accidental opening of a portal to each other's dimensions was the way to go. But then, as I thought more about it I began to turn to some of the crossover kings: Marvel and DC comics. If you look at the majority of the crossovers that they've had with one another (specifically, the Marvel vs DC mini-series in 1996) the universes crossing over with one another isn't accidental, but deliberately caused by external forces. In the case of Marvel vs DC, the instigators were two cosmic brothers, each holding power over one of the universes respectively, who purposefully merged the worlds together. This is what I think would make a good storyline for a Sonic/Mario crossover. Imagine there are two multi-dimensional, deity-like beings, one good and one evil. If anybody here is familiar with the Transformers universe, think of them as analogues of Unicron and Primus; the yin and yangs of the universe. But these "gods" were unable to directly and physically interact with the dimensions that they watch over. However, they
are able to influence the inhabitants in order to nudge them in the direction of doing their bidding. Now, the evil god thrives on chaos and destruction, and sees great potential to fulfill these feats through the means of the Chaos Emeralds in the Sonic-verse. However, since Sonic seems to keep them from falling into the "right" hands over and over again (which would be Robotnik's), he decides to give Eggman a little help in the form of our good old King of the Koopas, Bowser. Thus, he summons Bowser to act out his wishes (possibly by sending a herald to speak for him, ala Silver Surfer for Galactus) and tears a hole between the two worlds, allowing the King and his troops to march through. However, the benevolent god sees that he must act quickly in order to stop these deeds from being carried out, and thus alerts Mario and Luigi in the same manner and sends them into the dimensional tear to stop Bowser. Anyway, it's just a preliminary idea and it could take any number of forms springing from that premise.
I saw somebody mention earlier the Spyro/Crash crossover for the GBA. I've always figured it would work much like that would, and also a bit like the two Legend of Zelda games, Oracle of Ages and Oracle of Seasons. But I'll get to that in a second. First, I want to point out the major flaw in what could have been a very decent crossover game, Spyro and Crash. The big mistake that they made with these games, is that they changed the gameplay styles of each game to fit one another. If you've ever played both Spyro the Dragon and Crash Bandicoot, then you'll realize that they have very different feels to them. Crash is usually restricted to the ground and pretty much has two moves, spinning and jumping, and is restricted to levels which are, for the most part, a straight line. Spyro, however, focuses much more on flying through the air, dashing and ramming enemies, breathing fire, and so on and so forth, as well as being given a much larger field of movement which allows for point-of-view changes and running in circles and all that good stuff (if you like to run in circles). However, for the dinky little sidescroller crossover, they toned both the characters down, specifically Spyro, in order to make them have similar gameplays. What resulted was a game that flopped because it had almost nothing to do with the predecessors of each series aside from the character images. What they
should have done, was let each game keep their individual gameplay styles. Like the aformentioned crossover, Mario and Sonic would each have their own cartridge (or disc). However, instead of trying to make them similar to one another and thus making them completely alien games, the Mario game would play like a Mario game and the Sonic game would play like a Sonic game. The only thing that matters here is storyline, in my opinion. Each game would tell a different aspect of the same story with the title characters crossing pathes every so often, much like the A and B games of Resident Evil 2. You can choose which one to play first, Mario or Sonic, and after beating that scenario you would throw in its counterpart and see the otherside of the story shown from the other character's perspective. Do you see what I'm saying?