I think Nintendo is leaving those months to third party. April already has insane amount of third party games. While may has only one (AC3), it's likely that many announced but not dated titles can come out in that month. We got Resident Evils, FFCC, Saints Row and so on. I expect to see not yet announced third party games for May too.
Emily has already said that Labo should be around April and that it could be interesting.