SID and Paula won't be implemented anytime soon.
For SID the memory limitation of the C64 + variants and the fact that it's the machine to use the chip, and that the .SID format already exists.
For Amiga, Delitracker custom formats (as supported by UADE) is currently a better option than VGM. Also, the chip reads from RAM which will make things harder for VGM logging as the log will snap up everything from code to sound driver state. This is also a problem with SNES.
As for the Casio chip, we probably won't support it unless there's an interest in logging files from that machine. In that case, you should do like
eito and prepare a fork of VGMPlay + vgmtools to actually make a proof of concept first. In fact, you should probably do that for every chip you propose.