PDF/A-2
PDF/A-2 carries over provisions from the ETSI/PadES standard. (source)
PDF 2.0
ETSI will feed these European-specific elements back into ISO for inclusion in the next release of the PDF standard, ISO 32000-2. (source)
Will PAdES be mandatory in the new PDF 2.0 standard? Or will PDF 2.0 be compatible to PAdES and other signature formats will be allowed, too?
Is PAdES mandatory for PDF/A-2? Or are other signature formats in PDF/A-2 also allowed?
PAdES is not mandatory. In particular the name "PAdES" is not even mentioned in the PDF/A-2 specification. Merely some requirements have parallels to PAdES requirements, e.g.
When computing the digest for the file, it shall be computed over the entire file, including the signature dictionary but excluding the PDF Signature itself. This range is then indicated by the ByteRange entry of the signature dictionary.
This is also a requirement introduced by PAdES signatures, in ISO 32000-1 this merely was a recommendation. De facto, though, this had been made a requirement by Adobe Reader long before.
Although ISO 32000-1 also allows the value of the Contents entry of signature dictionary to be a DER-encoded PKCS#1 binary data object, that format is not recommended.
This effectively recommends the use of PKCS#7/CMS signature container based PDF signatures. This also parallels PAdES which actually goes a step further and requires it. But naked PKCS#1 object based PDF signatures are not fashionable anyway, not even in the plain PDF world.
ISO 32000-1:2008 allows the inclusion of one or more RFC 3281 attribute certificates to be associated with the signer certificate. However, a conforming writer should not include them as they are not widely supported and hence use of this attribute will reduce interoperability.
There is a similarly formulated recommendation in PAdES part 2 (Basic Profile). In PAdES part 3 (PAdES-BES and PAdES-EPES Profiles) attribute certificates are even forbidden. But nowadays attribute certificates in general are not in fashion anymore.
Thus, there are parallels but that's it.
Will PAdES be mandatory in the new PDF 2.0 standard? Or will PDF 2.0 be compatible to PAdES and other signature formats will be allowed, too?
As PDF 2.0 has not yet been published, this strictly speaking is speculation.
In the last draft I could read, though, PAdES signatures have been added to the existing formats and only adbe.pkcs7.sha1 signatures have been deprecated. As SHA1 (which is used in that format at least once) has forfeited trust, this format should not be used anyways, even in current PDFs.
Thus, neither PDF/A-2 nor ISO 32000-2 enforce PAdES style signatures, adbe.pkcs7.detached style signatures are still valid options. If interoperability and long term validation features are of interest, though, PAdES most likely is the better choice.