Selena's Whip - Handy in the right set, though it's a narrow hoser. I think it might be better if it just gave you one Gold but triggered off hitting any enemy. The card still holds flavour that way.
Riddler's Game - This is worse than 1UB draw 3. I'd prefer if it didn't backfire and instead put those cards into your graveyard, so in the right deck it would still work for you. That fits the Riddler's style in that the opponent never fully wins.
Arnold Wesker - I agree with Creedmoor in that it's a great design, but feels wordy. How about something simplified, like:
Arnold Wesker 1WB
Legendary Creature - Human Wizard
Mythic Rare
When Arnold Wesker enters the battlefield, create a legendary 2/2 Construct artifact creature named Scarface. T: Target creature you control named Scarface gets +2/+2 and gains indestructible and menace until end of turn.
2/2
Great design there! It suits the mythic rare title as it's something you would only really want to play 1-2 of. That it relies upon what you're opponent is doing makes it feels quite manipulative and very UB.
Demented Knight-ErrantR
Creature - Human Knight
Whenever Demented Knight-Errant attacks, put a mirage counter on target non-creature artifact an opponent controls. It loses its abilities and becomes a 4/4 blue Giant Illusion creature with "Whenever this becomes the target of a spell, sacrifice it."
2/1
You still must protect Don Quixote from the windmills, but now any shock is good enough.
It's a cool effect, preferably as a mode on a spell like:
Play on Weaknesses 1UB
Sorcery
Rare
Choose one:
* Gain control of target creature with power 2 or less.
* Gain control or target planeswalker with 2 or less loyalty counters on it.
Inner-Eye Explorer - This is a fun little Jny/Spike card. Great design. I wonder if a U cost 1/1 could help it interact better with Fetchlands.
The Unquestionable - Change the last wording to "When you shuffle your library, look at the top five cards, then put them back in any order." You could even try something simpler like Scry 3 if you wanted.
Renzala - It's a fun guessing game mechanic. You could cost her more and make her a Sphinx with flying for the flavour, or maybe as a Pirate gambler. She works best in a low creature deck with bounce and counterspells, so perhaps she's best as a big beater herself.
Sucram - He looks very deadly... until you realise you only really get one shot at being able to semi-draw a card for R. I like the tension here.
Send to Banishment - I don't get how the pieces of the card work together. Also, why does white skip a turn? That was last done in Chronosavant which should really have been a blue card.
Ruminate - Meditate that's better for 3+ players could probably be a 1UU instant at rare.
I like the enchantment exiling version, though make it more like Banishing Light so you can get it back if Plan A fails. I also believe 3/4 would be fine and move it just out of range for much burn.
Selena's Whip - Handy in the right set, though it's a narrow hoser. I think it might be better if it just gave you one Gold but triggered off hitting any enemy. The card still holds flavour that way.
Riddler's Game - This is worse than 1UB draw 3. I'd prefer if it didn't backfire and instead put those cards into your graveyard, so in the right deck it would still work for you. That fits the Riddler's style in that the opponent never fully wins.
Arnold Wesker - I agree with Creedmoor in that it's a great design, but feels wordy. How about something simplified, like:
Arnold Wesker
1WB
Legendary Creature - Human Wizard
Mythic Rare
When Arnold Wesker enters the battlefield, create a legendary 2/2 Construct artifact creature named Scarface.
T: Target creature you control named Scarface gets +2/+2 and gains indestructible and menace until end of turn.
2/2
Haha this is a fantastic flavourful design!
Play on Weaknesses
1UB
Sorcery
Rare
Choose one:
* Gain control of target creature with power 2 or less.
* Gain control or target planeswalker with 2 or less loyalty counters on it.
This gives the mechanic more utility beyond Legacy, and gives more for each colour to do (i.e. green).
Tree of Knowledge
3G
Creature - Treefolk
Insight G
3/3
Good thinking about the wording! This could be either silver- or gold-bordered, given the effect doesn't yet exist. Nice.
The Unquestionable - Change the last wording to "When you shuffle your library, look at the top five cards, then put them back in any order." You could even try something simpler like Scry 3 if you wanted.
Sucram - He looks very deadly... until you realise you only really get one shot at being able to semi-draw a card for R. I like the tension here.
Ruminate - Meditate that's better for 3+ players could probably be a 1UU instant at rare.