Most would probably tell you the Wii does not use ROMs per se -- ROM tends to be reserved for things with carts. Anything with discs tends to be an ISO. That said the Wiiware/virtual console stuff might count as such and we know what you are asking for.
Still there are two main things Wii games come as.
1) The basic optical media most games came as. In an ideal world it would all be one format but you might find things already stuffed into WBFS out in the wild rather than the main iso format that the rest of things settled on.
2) The Wiiware and Virtual console stuff, much of which did not appear on discs and some discs also contain these. Most would know these as WADs, not to be confused with the Doom format of the same name/extension. https://wiibrew.org/wiki/LibWiiSharp example tools are probably what you would be pointed at for an unpacker, u8mii to handle the u8 compression commonly used in them.