Hi Wladimir,
If the motivation of the SPV wallet is to radically extend functionality, as in my case, then the index is specific to the added features and the subset of the blockchain that is of interest for the wallet.
As you also point out, adding huge generic purpose indices to core would rather discourage people using full nodes due to excess requirements.
I believe nothing would add more to the core’s popularity as a trusted background node to SPV than full validation at lowest possible memory, disk and CPU footprint.
Serving headers should be default but storing and serving full blocks configurable to ranges, so people can tailor to their bandwith and space available.
Tamas Blummer
Bits of Proof