Not everything, depends on the system. On the NES, for example, you can only mess around with RAM if you find ACE, but if the cartridge is using CHR ROM, whatever you create with ACE must still use the only tileset available to the game. You can get creative drawing graphics with a fixed set of tiles, but you'll be constrained nonetheless.
There may be other constraints for other systems. I don't know the SNES architecture too well, but I assume even with ACE you're still limited in various ways to the constraints of the cartridges.
SNES does not put video memory on CHR-ROM, it's all writable RAM. The restriction is total RAM avaialble (including cartridge save ram), possibly calling functions or using data from the original ROM.
There may be other constraints for other systems. I don't know the SNES architecture too well, but I assume even with ACE you're still limited in various ways to the constraints of the cartridges.