In a nutshell they
need other IPs for that IMHO — although as I said I'd prefer a bit more coherent Mario worlds as well — like Zelda, Metroid, the Monolith
games and so on or
of course new IPs... They could have expanded
during the last 10 years but they did choose not to, as far as we can tell (except acquiring Monolith)
during the «Iwata era».