I welcome any card or setting suggestions, and polite criticism. As the sole person designing the set, it's a lot to do (as you all well know!), and I thought opening it up for a bit of discussion would help me drive forward toward completion.
Note that I do already know that this is likely not a set that Wizards would ever print. This is something I'm making for my own personal enjoyment.
By the way, I have nobody with whom to playtest, since getting Jon to help out with that sort of thing is an utter chore and makes neither of us happy. So if anybody ever feels like doing so once the set is more fleshed out, I would be much obliged and very happy to hear the results.

FWIW, I had a look over the mechanics and skeleton, and it looks pretty good to me. I'd not realised how well it was coming together from seeing individual cards.

Thanks, Jack.
The one thing I'm most unsure about right now is React. I'm starting to wonder if it's a bad idea.

A few brainstormed ideas for your "blue feel" cards:

• Activated +1/-1 and -1/+1 (probably )
• Mill effects ()
• Colourshifting Spreading Seas into other colours. I think any colour can turn things into its own basic land type (Evil Presence/Tainted Well, Celestial Dawn, Magus of the Moon), so put it wherever it'll matter most - perhaps green if that's getting a land-serpent. Alternatively, for a slightly bluer feel, green could probably do Phantasmal Terrain / Dream Thrush.
• Similarly, I think green is the one most likely to do creature type changing or colour changing (Swirling Spriggan).
• Tutoring for artifacts ()
• We don't normally see "high flying" (fliers that can only block fliers) outside of blue, but I don't see any reason why it shouldn't be seen in red or white.
• For a black card-draw spell with a blue feel, perhaps it could be a variation on Skulltap or Night's Whisper, but sacrificing something else. Perhaps black could get Perilous Research?
• Another random idea: a black-shift of Wistful Thinking? That card was blacker than blue normally gets, but it's also slightly bluer than black normally gets, which is kinda what you want for this set.
• Effects like Prodigal Pyromancer and Reverberate that used to be blue but have moved to other colours now should be definite shoo-ins for this set.

Those are all excellent suggestions, Alex. I'll go over them thoroughly when I get home from work.

How do you make bullet points like that, Alex? I still haven't figured it out.
1. +1/-1 seems great for red, and perhaps I'll stretch -1/+1 into white. White already gets shieldbreathing, and it would certainly make sense for white to reduce its power if it was trying to be pacifistic.
2. Black will definitely be getting some mill.
3. Ooh, Dream Thrush would make a great shift to green, especially given that in my little blurb about the different color's themes, I mention that green's way of surviving is promoting the spread of life and community. A green creature that turns things into forests might be a good way of showing this.
4. For some reason I always think of red as a color that changes colors, and I have no idea why. Perhaps it's my artistic red civilization from Onora that polluted me.
5. White often gets equipment tutors, and it gets artifact recursion well enough. Something like Trinket Mage could probably make sense in white.
6. A high flier in white is a great idea for one of my common "blue feel" white cards!
7. I was thinking of giving something like Compulsive Research or Thirst for Knowledge to another color. Perilous Research sounds like a great shift, too.
8. I would never have thought of Wistful Thinking. That's a good one to look at.
9. I always forget that Prodigal Pyromancer used to be blue, since it makes so little sense with blue's current color pie. Char might be a good reprint as a color-shifted Psionic Blast. I thought about Reverberate, but I think the slot I would put that in is probably filled by Phase Shift.

Bulleted lists are done in Markdown by starting each line in a paragraph with *, where the paragraph is set apart from preceding text by a blank line.

1: I was actually thinking of a creature with both C: +1/-1 and C: -1/+1, as a Morphling reference. I'd say Torchling had a "blue feel" in the way you're going for.
7: That sounds risky. I don't think a nonblue Thirst for Knowledge would fit anywhere else. I suppose Compulsive Research could be a Harmonize variant.
9: Ah, yes, I agree you wouldn't want both Reverberate and Phase Shift in the same set. Maybe one in set 1 and one in set 2? To make a more blue-feeling pinger, how about a colourshift of Fledgling Mawcor (minus the morph) into red? Or Thornwind Faeries, I guess, but Fledgling Mawcor has the Anaba Shaman stats to help it feel more red-suited (while still feeling a little blue).

1. You know, this might be the perfect set for the long sough-after White or Black Morphling.
7. I could possibly see a red Compulsive Research variant, but I think Perilous Research is a great shift. I'm thinking black can't quite get it straight up because it's not allowed to get rid of its own enchantments, but it could get something similar, or red could get it.
9. A flying pinger is a great callback blue reference.