As far as disc format goes, A modified Bluray format is probably the cheapest and most likely disc Nintendo will use for Wii 2. They wouldn't have to pay anything to the Bluray Consortium in terms of licensing fees, since it wouldn't be a Bluray disc in the truest sense.
How possible is it for Nintendo to go for a modified HD-DVD disc format? The biggest problem with going for modified Bluray is that someone will probably find a way to hack a Bluray burner and make copies and images, as it happened with the Wii Optical Disc despite its safeguards. With HD-DVD, that would be near impossible, since no one makes burners or discs for that format any more. Also, discs would be 30GB, maybe 45-51GB if they go triple layer - Final Fantasy XIII and MGS4 are the only two not-Sony games that fill a dual-layer Bluray IIRC.