Personally, while I liked Shake It, I would definitely prefer a sixth game by an in house Nintendo studio, with more of the ideas that made the earlier games good.
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If they broke from the formula with Shake it I doubt they'd go back, but Mario Kart 7 did that.
I always think just numbers beyond a certain point is a bad thing. They showed Fast and Furious 7 for a week on bus adverts before I noticed it was a new thing. This is because you're only changing one character in the title and it didn't pick up in my brain it was different. Could be cause I don't watch F&F tho.
I think they used Shake It! Because they wanted to reach a new audience, a one which had never played Wario Land. Big numbers scare people off. Should I have played 1-4 first? I'll just not risk missing the plot. It's not Wario Land 5. It's Wario Land shake it! which if you didn't know better could be the first in a franchise.
I doubt they'd go from that to just 6. People would be like where's 2-5?
People always refer Wario World as a Wario Land due to been a platform ( + the beat 'em up) gameplay and a story-line that evolve Wario getting his riches and castle back.
And I also agreed with @Bobbins about Nintendo breaking the "number formula" and leading a audience who never heard of the series.
But it's also bad because that would lead the new audience to think that just exist one Wario Land. It just Wario Land: Shake It!/The Shake Dimension, if they could make like Wario Land 6: Shake It!/The Shake Dimension, that could make the audience rethink about it and say "Hmm, Wario Land 6? There was previous games? I better check it out!"