all cd/dvd based consoles could still have everdrive support through memory cards and card slot, simply make the consoles memory card have a slot to put sd card with loaded everdrive roms in it.
It's not that simple, sadly. The console would need to be able to boot from the memory card or card slot, which as far as I know, none can do normally (there'd be no advantage for the manufacturer to deliberately add such a feature). If there's a flaw or vulnerability in the console's operating system then it can be possible to force the memory card to boot as a result of this, but that's not easy to do, and you usually need another way to run homebrew party software on the console first so that that software can then make the memory card bootable.
The PS2's ability to boot from the memory card (or hard drive) is only possible because some very clever people spent a lot of time doing some very hard work to find and exploit a suitable vulnerability, and few consoles have a user base as fanatically devoted as the PS2, sadly.
I would like to see some device created by Krikzz that fits into the PS2 network adapter port and does everything a hard drive does for it.
I know that it's currently possible to scrounge for a used ps2 network adapter and an IDE hard drive and accomplish what an Everdrive can do but I would love to see a quality device made by Krikzz that does it without having to fuss and dig around for legacy adapters.
A PS2 with a hard drive is fantastic, but it's not perfect, as not every game works from hard-drive (though compatibility is still being improved, and it is now very high), and some games need some experimentation with some settings (in the boot loader software) to get them to run properly. It's not like an Everdrive (or even a modded XBox) where every game works fine.
Having said that, though, it's difficult to see how an SD card would make a better storage medium for the PS2 than a hard drive, as PS2 games are much larger than any games for any of the consoles that so far the Everdrive range caters for. Some games on the PS2 are more than 5GB in size, and I've filled my 250GB* hard drive on my PS2, and I don't have that many PS2 games (i think it was around 72 when I last counted, but that was a year or two back).
Even if you could get larger than 250GB SD cards, they'd cost a lot more than a hard drive of the same capacity would. Though then again, since the PS2 + network card needs an IDE hard drive, which can be hard to find as they began to be phased out a while back, then an Everdrive that used SD cards, or even SATA hard drives, might be tempting. BTW, you can use a SATA to IDE converter and a SATA hard drive, but sadly a lot of them apparently don't work with the PS2, so it's not too straightforward there.
* I could be wrong here, but I seem to remember that I could only access about 220GB (or 230GB?) of the hard drive, as to use the rest I'd need to reformat it, or use a new BIOS, or something, and by the time I found out about it, I'd installed most of my games, so I didn't want to go to the trouble of having to reinstall them, so I decided to just 'lose' the extra GBs of hard drive space.