I don't think they'd release three huge platformers that close to each other. Also, Nintendo hasn't been releasing as many Mario spin-offs lately, which is why that's what I think it is.
It's a long shot, but it's been a fair old while since the first. They can just get the voice work out the way then sit on it for several more months.
A new Golf game would be my choice of spin-off, though a ninth (or eleventh, counting Advance and DS) Party game is probably more likely.