@Truss: I don't know, but Commodore 65 is intended to be C64 compatible at some degree at least ... So about the Mega65, if it wants to carry the original concept of C65 further, should not forget about the roots. However that's true: even the original C65 was not meant to be a 100% compatible C64 in C64 mode (well, just the CPU is not the very same - think about illegal opcodes -, and so on), that's the other point. And I think, C65GS/Mega65 has already a great work to even lighten this limitations of the original design (for example: think about the incompatibility caused by the different behaviour of read-modify-write opcodes of 65CE02 core, which is already solved - if I know well - for Mega65).
But I agree, that only "an FPGA C64" it wouldn't be as interesting as Mega65 is, there are projects already like that. C65 is an interesting (however it was way so late to be developed by Commodore, IMHO) machine, and with the original concept, and a proper case/keyboard/etc it really feels as a brand new (and quite modern) 8 bit machine, which is a great feeling! It's just another - though very important! - point, that Mega65 has even more developments over the C65 features, for me, this makes it even more interesting and a "must have" item :) Personally, I felt something similar with C64DTV, that I have something which "looks like" a 8 bit stuff (and not only a software emulation), which carries the original concepts further, and "just can be bought" and ready to be discovered, etc. Mega65 is even better with a proper case, and the general feeling about being a "real computer", which is not so much the case of C64DTV (well, it's just a "joystick", you have to hack it, etc).
But for sure this was *ONLY* my personal view, nothing more.