What would you want from a good emulator today?
Z80 emulation... Good Z80 emulation. TBH, really good emulation starts with the bus and works inwards
ULA emulation + those extra bits that get used all over - well, some of them anyway.
We'd want ROM and RAM chip select decode and full I,R,RFSH cycle decoding/interpreting or the system just wouldn't work.
TV emulation - emulate interlacing effects and much more (pixel blur, even warping)
Ear and Mic sockets need good emulation too (I want sound damnit)
Back when this machine came out there were TINY ROM cartridges and everything else came on tape (it was very cheap). If you wanted fast and big back then you could've used floppy discs or very small amounts of Flash (4K). Painful.
Software, well, a decent multi-pass disassembler in the "IDE" would be perfect as would be the ability to load up pre-built RAM/ROM images and give them a go.
A built in assembler would be essential although wouldn't give you that authentic feel! We'd supply an assembler on ROM (additional 8K ROM). TBH, It's the only way to go - A really good cartridge would have additional RAM so we could leave main memory untouched... Can we run the screen from an address < 16384? (my feeling is "NO")
Memory watch windows with some reconfigurability for weird floating point types.
The ability to follow a 16 bit value as an absolute of relative address would be nice too.
You see, there's SO much to do!
Lastly, I'd like to be able to use the emulated machine font for the GUI. Now... that's hard!