Incredible
I Love this series (too lazy to write a review for all of them) and I say keep going until you can't go anymore. This seires also has great potential for the way you team up Mario and Sonic. However, I do have a couple of suggestions:
1: Think about moving the series to Sonic's world. The Mushroom Kingdom and the surrounding lands are OK but I would like to see the action move to Mobius.
2: Bring in some more new characters. It would be great to see StarFox and his team shoot down Metal Sonic or have Spyro torch Luigi and Crash spin away the Koopa Bros. Or even Link can lend a hand with the Triforce. And,
3:Make a Super Mario Bros. Z Budokai. Have a fighting game where you can finally kick some Koopa ass and get the crap beaten out of Sonic by Shadow using a Chaos Control. If you do end up making a fighting game, be sure it has a Story and a VS. Mode.