@gpz: Yes, you can put anything on a cartridge, I meant in terms of having the address lines properly decoded. Of course, SIDs being mostly write-only they are easier than some other things to put there. Sorry for not being entirely accurate there.
@zeha: We haven't made a final decision on composite output. We can in theory generate it directly from a very rapidly switched FPGA pin. There are some complications, but we are still considering it.
@mega65lover: Well, you might get more than you wish for there. The trouble will be if you need lots of things on the cartridge port at once, as it may not be possible to have them all connected at the same time due to various complexities.
@adtbm: re tape and user port, we expect that there will be an adapter for the expansion port that will allow it to be used instead as user port + tape port, but it might not be available immediately.
Regarding operational voltages, we have voltage level converters where they are needed, so all chips that expect 5V signals will get them. 9V/12V for SIDs we don't yet have the final solution for. If they are in a cartridge, then they might need a power supply lead of some sort to the cartridge. If we put internal SID sockets, then we will supply at least the 12V signal.