Well, Bitcoin Core has a very simple format. You don't need any developer support to understand it:
label=%s # addr=%s (in-use keys)
reserve=1 # addr=%s (for reserve keys)
change=1 # addr=%s (for change addresses)
Time format is ISO8601 (%Y-%m-%dT%H:%M:%SZ), key format is WIF (importprivkey format).
That's certainly an example of user-friendly format to start from. The point is: despite being that simple, most software seems reluctant to import it in an automated way.
I actually agree with you though, it would be great if different wallets had a standard export format. The first step would be writing a BIP. If the proposal is sensible I don't think much is needed to get wallet authors to implement it, as the need for this is kind of a no-brainer.
For the different deterministic wallets this is more tricky as they (currently) use different ways to generate the keys. If they all used BIP32 it'd be a lot easier.
Exactly. I had originally posted this question on
StackExchange where I complained about HD wallets in particular. The only significant barrier is defining some default derivation behaviour (e.g. 100k rounds of SHA256) and gap limit to reduce paper backups to just mnemonics. Creation time is a mere optimization and can be optional, but any custom parameter must be exposed no matter what. As to the branching structure BIP44 is a good candidate too and it defines a default gap limit of 20 that sounds reasonable for most users.