Is there any known [contemporary] languages getting more cases, more genders, dual number without having it before?
No. There is nothing like this that we can see right now.
Cantonese and other Chinese languages seem to have continued to grow in complexity since the branching off of Mandarin. This might be the best candidate for further inquiry, but I actually don't know much about it.
John McWhorter himself, here, seems to think (or at least he did in 2003) that the common notion that languages "naturally" loose inflections and other grammatical components over time is not accurate. The transformation of Latin into the 'simpler' Romance languages and the various transitions from Old English to modern English were likely not as 'natural' or relatively passive changes as most have blindly assumed.
If it is the case that all languages "naturally" decay in grammatical complexity, McWhorter asks Why then are there so many instances in which languages have not done this? Examples include over 500 Bantu languages, the native languages of North America, native languages of Australia, and even proper Sinitic (Chinese) languages (that is, excluding Mandarin) -- none of which have ever spun off a stripped-down version of themselves.
Smarter linguists looking past the idea that simplifications of language "just happen" are asking questions about what the exact mechanisms for such change might be. Consider this comment from the source cited above.
The evidence suggests that the post-Neolithic "punctuations" that Bob Dixon describes in human languages' timelines have often sheared away a degree of languages' "mess" as they were imposed on adult speakers and passed down in abbreviated form to succeeding generations. Mandarin's mysteriously compact four tones would be a similar case, it having been adopted by Mongol invaders while Cantonese and the rest of the brood mutated uninterrupted, developing the eight and nine tones typical of the "card-carrying" Sinitic language.
Thanks for the link. Still some questions: 1. “Cantonese and other Chinese languages seem to have continued to grow in complexity”. Do you mean grammatical complexity? 2. Why do you say “known [contemporary] languages”. Do you mean all languages (also extinct) with known grammar?
1) Yes. 2) Yes. To the extent I know, the accrual of “grammatical” prefixes, suffixes, and other linguistic markers that are micro-contextually determined on case, tense, gender, etc. have reached their apexes outside of written language. Therefore, we do not have specific examples to rely upon. But again, rest assured that we assume all languages have “developed” and therefore we are constrained also to assume that the accrual of ‘grammatical junk’ was, in fact, also a process. However, there might be concrete examples to find looking more closely at the Chinese languages.