Hello, I would appreciate any commentaries on how to tune up the mainboard of this deck while keeping its flavour at the same time as keeping it reasonably competitive, and avoiding falling into a meta deck.

The idea behind this deck is to play a medium aggressive/controlling grixis deck focused on the "steal" and "play the opponent's deck" theme, so piloting it will bring a different approach in every match, according to what you steal. This way, I can get some kind of infinite replayability from it, making it more worth to invest some big bucks on it.

It starts the early game with a typical grixis control build, using discard, counter and removal spells like Fatal Push, Inquisition of Kozilek (i choose it over thoughtseize because this deck's manabase is already heavy enough on self damage), Mana Leak, Countersquall and Lightning Bolt.

On T2, you have 4 options:

a) Play a Robber of the Rich to start applying pressure and building up your alternative hand with the opponent's cards. The robber's restrictions are easily overcame since the deck packs plenty of rogues and cheap spells. So if it gets killed on the turn it attacked, you can always cast the exiled cards later with the help of a Brazen Borrower or a Rankle, Master of Pranks.

b) Play a Kitesail Freebooter following along the hand-control plan, which gives you a pseudo-discard while gathering intel about the opponent's hand. It tends to attract some hate, which can come handy on the first match, as it acts as a lightning rod to clear the path for your T3 Thief of Sanity.

c) Gain some tempo with Brazen Borrower's petty theft, bouncing an annoying permanent (specially enchantments, since we don't have another answer to them except for discard)

d) Counter a spell with Mana Leak.

On T3, you can either drop a Thief of Sanity or a Brazen Borrower.

The first one, along with the Robber of the Rich, is one of the main engines of card advantage. Since we are talking about modern, most spells should be cheap to play. Also, it can serve the purpose of eliminating important combo pieces.

Otherwise, the Borrower poses a nice clock on the air.

Bedevil and Kolaghan's Command as a catch all (except enchantments) answer to anything.

T4: Rankle, Master of Pranks is a flying rogue; so, on one hand, it helps casting the cards exiled with Robber of the Rich; and on the other hand, its evasiveness helps cheating a ninjitsu attack with Fallen Shinobi.

Fallen Shinobi: God do I love this card. It is complemented with the rest of the crew so well. Since almost all your other creatures have flying, the probability of them being left unblocked is substantial. Thus, you can easily sneak a Ninjutsu on your enemy, hitting for 5 and casting 2 free spells. Also, if you bounce back a Brazen Borrower, you can reutilize its petty theft. Talk about value.

5: Nicol Bolas, Dragon-God steals your opponent's planeswalker's abilities, while generating card advantage and being a wincon on its own.

T6: Dragonlord Silumgar dives in to steal their already ticked up planeswalker or finishing creature to close the game.

Alternatively, a Dragonlord Kolaghan is also a great finisher that plays along the line of this deck. With so much removal, a soft lock is guaranteed when he hits the board.