Thanks for the review GenericZero, that's some decent input!
Okay, so here's my review:
Spell Tax: I can dig this for the most part, but I don't like the idea that ANY white deck could have this ability when white should suck at drawing cards and shouldn't have or need access to counterspells unless it is teamed up with blue.
Cantripping is fine in white, and mana taxing is in flavor as well (though usually affecting permanents). I think this card is sweet obviously
Mindreave: I would never want to play this, personally. The variables are too high and it isn't skill-rewarding. I get that U/B is hard to design for but we can do better than this, can't we?
Alright, how about this?
Alter Memory
Sorcery
Look at target player's hand. You may choose two nonland cards from it. If you do, that player reveals the chosen cards, puts them on the bottom of his or her library in any order, then draws two cards.
Terra Snapper: This needs a bit more going for it. Make it a 2/2 and possibly give it natural haste instead of conditional? Might be fine as is, but I like having 1 more point of toughness, personally.
Yeah, +1 toughness seems fine. I like the tension on conditional haste though. Do I pitch this land or wait until I'm sure I got excess lands?
Terra Snapper
Creature - Elemental Turtle
Discard a land card: Put a +1/+1 counter on Terra Mauler and it gains haste and trample until end of turn.
2/2
Sundering Wave: A better Naturalize still has the same issue as Naturalize. Too narrow and while it answers two card types that lack many answers, it needs to do more than that. People like Reclamation Sage and Quasali Pridemage cuz they give you more than a narrow effect. Possibly just make this a 2/1 with flash?
I don't know, Disenchant is a legit card in cube as far as I'm concerned. Though if you really want to make it better...
Sundering Wave
Instant
Choose one — Destroy target artifact; or destroy target enchantment; or destroy target artifact and target enchantment. You gain 4 life.
Incidental life gain is the shiznit!
Mirrorgaze Soul: Give it flying, maybe make it an elemental since we don't usually see spirits in red.
Then again, we don't see a lot of flying in red either, do we?
Also, the ability is pretty pushed anyway, I want to follow this up with
Cone of Flame!
Cockroach Overlord: I like it, but can we make it a 3/3? It feels a lot better as a 3/3. Perhaps another creature-type change as well.
Yeah, the creature type is silly. Could easily be a Zombie or something to make for better type support. Though, picture it, Cockroach Overlord, COCKROACH OVERLORD DAMNIT!
Zombie Cockroach Overlord
Creature - Insect Zombie
You may cast Zombie Cockroach Overlord from your graveyard.
Zombie Cockroach Overlord enters the battlefield with three +1/+1 counters on it if you didn't cast it from your hand.
Counters remain on Zombie Cockroach Overlord as it moves to any zone other than a player's hand or library.
3/3
War Spell: It costs 4 mana in a VERY aggressive color combination and doesn't do anything on an empty board or do anything instantly. I DID find myself inspired by it, though....
Helix Trenches: 2 hybrid mana. R/W. Enchantment. Whenever a creature enters the battlefield under your control, choose one: deal 1 damage to target player OR gain 1 life.
2 colorless, Sacrifice a land: Put a 1/1 soldier creature token onto the battlefield.
But direct damage isn't white and life gain isn't red, so this doesn't feel like a hybrid card at all to me
I put War Spell at four mana, so that you can follow up your early drops with a curve topper. I think you could change the trigger to start of combat though to make it have an immediate impact. How's this?
War Spell
Enchantment
At the beginning of your combat step, put a strife counter on War Spell.
Attacking creatures you control get +1/+0 for each strife counter on War Spell.
Giant Ophidian: U/G seems to have enough Scroll Thieves and whatnot...Why not a 3 mana 1/1 dude that gives us explore? Seems fine by me. Or even a 4 mana 2/4 that gets a +1/+1 counter every time we draw a card?
3 mana? We can do better to improve upon the almost playable Coiling Oracle!
Coiled Explorer
Creature - Elf Snake Scout
When Coiled Explorer enters the battlefield, draw a card, then you may put a land card from your hand onto the battlefield tapped.
That's not really a mono-blue ability though, so this ability is skirting the borders of what hybrid cards should do as far as I'm concerned. There is that blue
Harrow on a stick though from Time Spiral block...