So I'm new to this particular forum (I've been on the WOtC one for a while) and I'm going to display some cards that I've made:

Court Denizen
Creature - Human Advisor
Legendary Creatures can't be blocked.
1/1

Monstrous Tyrant
Creature - Ogre
Hero's Reward - When ~ leaves the battlefield, each opponent draws a card.
3/4
It has a high price on it's head.

Street Assasin
Creature - Human Assasin
: Exile another target tapped creature.
Hero's Reward - When ~ leaves the battlefield, each opponent may put a creature exiled with ~ onto the battlefield under their owner's control.
1/1

Thick Snow
Enchantment
Creatures loose haste.
Attacking creatures get -1/-0.
Hero's Reward - When ~ leaves the battlefield, each opponent gains 3 life.
Uh, thanks for clearing the snow!

Fickle Alleigance
Enchantment
Whenever a planeswalker is put into a graveyard from the battlefield, it's owner return's to the battlefield under the control of one of their opponents of their choice.
Insert awesome Nicol Bolas flavour text here.

Exquisite Sniper
Artifact - Equipment
As long as equipped creature is blue or black, it has " : Destroy target tapped creature without haste or vigilance. You may activate this ability only if this creature hasn't taken any damage since the start of your last turn and only if it is not combat.". Otherwise equipped creature gets +2/+0
Equip
Put that thing down! No, Borborygmos, it is not a club. You're going to break it.

Hey, welcome to the forums! I'll be brief here as I don't have much time atm.

Court Denizen - I really like this card mechanically, but I'm not really seeing the flavour come across. It seems like a common/uncommon for a retake on the legendary matters theme - perhaps a super high society - political backstabbings, duels and ballrooms, I'm on board already, but the card itself wants some flavour text to paint the world better.

Monstrous Tyrant + Street Assassin - I've seen variants on this mechanic before (they normally call it bounty though) and the problem with them is they're hard to balance in play effectively, as the same reward can mean very different things in different matchups. Solid design, although the Assassin fails flavourfully if the creatures can come back.

Thick Snow - Interesting to see the mechanic on an enchantment, even with such a minor effect. Card itself is a little outclassed by other white and blue cards designed to fill the same whole, but is decently balanced.

Fickle Allegiance - This is super niche. Cool, but not really playable as it requires a very specific type of opponent and sort of prevents you playing your own planeswalkers. Make it one sided and give it more of a villainy feel.

Exquisite Sniper - A Sniper is a person who snipes, not a object. It has too much going on, not all of which makes sense. Why can blue creatures use the weapon like assassins? Why does it not work like that for white/red/green creatures? Restrictions all seem a little arbitrary, just choose one (probably the not in combat one) and stick with it.

Welcome to the boards.
I think Morgortha pretty much hit all the same critiques I would've made. So um just double those I guess.

Welcome to the new boards!

FIRST - If you're designing cards for a set, you really need rarities on the cards. Am I supposed to see Court Denizen every game, every other game, or once per Limited event? Rarity helps define that.

Court Denizen - As stated, the card is interesting provided you do have a "legends matter" subtheme. However, I kind of feel that the card would work better as "Legendary creatures can't be blocked as long as Court Denizen is attacking."

Hero's Reward - It's a fine idea for a drawback, but is this a mechanic you want appearing on 10% of cards in the set? If the answer is "no", then perhaps this shouldn't be an ability word, and should instead just be confined to a single Uncommon cycle.

Street Assassin - Very powerful. Too powerful? Can save your team from Wrath-effects, and totally neuter an opponent who has no direct removal. Should at least have a "tap" in the cost, and make the "reward" more than just "you get back what he took" (there is no "extra" involved).

Thick Snow - Do creatures loose haste, or lose haste?

Fickle Allegiance - Practically unplayable, unless the set has a preponderance of Planeswalkers... which it likely shouldn't. Not only practically unplayable, but it doesn't hit any player demographics outside the "I hate planeswalkers" demographic.

Exquisite Sniper - Why is this giving such a weird bonus to blue/black, but such a standard bonus to others? I'd say just have this give any creature +1/+0, but turn only blue or black creatures, or perhaps only LEGENDARY creatures, into Royal Assassins. However, you shouldn't probably have more than 1 Royal Assassin-type card per set, so either this or Street Assassin need to go.

Instead of creatures losing haste, why not have them enter the battlefield tapped?

I do like the flavor text on exquisite sniper though.