@Slith: The kind of design where they just remove a -1/-1 counter fom themselves (and preferably move it to another creature) when they damage a player would have worked fine. Or just the kind of thing
Thrummingbird does. There would have been lots of possibilities to stick to the "growth" theme. I hope they'll be around again the next time we see New Phyrexia, they already looked pretty creepy and kinda Phyrexian to begin with. Alternatively, I wouldn't mind seeing the Slith on their original homeplane. It's probably some alien place like Fabacin or Belenon or even Iquatana...
@Phyrexians: I love Phyrexians, and Old Phyrexia had a huge part in drawing me into the game. It was like Christmas when they brought back Phyrexia AND poison in the same block. Though I hope New Phyrexia will evolve some more and become more like Old Phyrexia regarding the look and body shape of the creatures. New Phyrexia had a lot of creatures that were just corrupted versions of the races of Mirrodin and still had the corresponding creature type (humans, elves, leonin, vedalken etc.), and that was perfectly fine as it showed how they were corrupting the plane. But I've always been a fan of cards like
Phyrexian Slayer or
Phyrexian Reaper that just look like sick alien cyborg monsters with totally inhuman body shapes. Or things like
Abyssal Gatekeeper or the original artwork of
Phyrexian Rager and
Phyrexian Delver that use body parts of humanoids but rearrange them into completely different silhouettes. New Phyrexia had some of those, too, but I'd like them to move away from their Mirrodin origns to some extent and closer to some of the old stuff.
@Myr: Memnarch had all of Karn's research notes, which must have involved lots of blueprints, too. I'm still pretty sure it said somewhere the myr were a completely new design, but I can't point to a source for sure. On the plus side, there are still myr on New Phyrexia, they've just been phyrexianised. I guess they didn't even notice and are still doing their thing, though.
@Suq'Ata/Rabiah connection: I've heard of that connection before, but it makes me wonder: Do we know anything about the Suq'Atans' religion? Like, are they canonically Muslims or something? I think we all got used to the real-world weirdness of Rabiah, but Islam existing on Dominaria might be kinda weird. Not that that's going to come up again I'm sure...