Dimir Infiltrator provides the ability to transmute for a few useful cards from a toolbox in the mid-late game. This works well since the infiltrator has a habit of ending up back in your hand again after you use a ninja's Ninjutsu ability.
A toolbox could be comprised of just a few cards and still cover a nice variety of situations. Ideally, you'd like to be able to fetch cards that are good to A)answering a specific threat, B)Refill your hand, C)Disrupt your opponent to stay ahead if the game is going well.
Comments on some specific cards:
Snapback: bounce a ninja back to hand (possibly as a free spell) for an attack on the same turn, or bounce a defender. Snapback is nice, because you could bounce a ninja for zero mana and still have mana to use its Ninjutsu ability the same turn in some cases.
Tyrant's Scorn is a nice alternative to Snapback since it's a removal spell as well. That allows you to hide a bounce spell in the toolbox without spending a slot exclusively on bounce. This doesn't kill expensive creatures, so it's nice as part of a suite of 2 mana removal.
Profane Command: Finish off the opponent with direct damage, but also kills a creature. It can also use the fear mode to setup a game winning attack. This is a mana heavy spell, but maybe the versatility is worth it. It's an inexpensive card to acquire, so it seems worth a try.
Mind Spring or Pull From Tomorrow: Refill your hand next turn if you have good mana and no cards. For cheaper alternatives that can be cast the same turn, Chart a Course or Night's Whisper might be preferred.
Feed the Swarm: This gets an enchantment removal into the toolbox, but it's also creature removal.
Counterspell: Fetching counter magic can keep you ahead in a game you're winning.
Collective Brutality: Disrupt opponent's hand, kill a creature, finish off the last 2 life points to win, gain 2 life to survive an extra turn. The nice thing about this card is you draw cards with this deck, so you will sometimes have the extra cards to able to discard two and use all three modes of this card the same turn you fetch it. This card lets you sneak both direct damage and hand disruption into the toolbox on a card that is also creature removal and a combat trick.
Having a 2 mana spell that can deal 2 damage to the opponent will occasionally win the game, so Collective Brutality is nice to have in the deck. Sign in Blood is another way to add that ability while also giving you that ability to refill your hand a bit in other scenarios.
The combination of Sickening Shoal and Snapback could be nice, because they're both free spells that can remove a blocker. Having both in the deck would mean you can transmute for whichever one matches the color of a card you happen to have in hand to exile for the alternate cost.
