Yo-kai Watch 3 will show if the franchise has peaked or not. I'm not sure Level 5 will release it at summer again, maybe they'll wait a bit longer this time (or maybe not).
But I still think they'll have a new game this Summer.. unless their mysterious new IP takes YW's spot in July. I dunno.
And personally, I think the franchise will peak with YW3 (at the earliest). Gen transition will be critical, especially with Nintendo which will try to get YW on the 4DS asap.