For the most part, trying to stay Dimir is ideal and lets you draft the necessary pieces for your deck when they show up, but on a draft-by-draft basis, you want to be aware of the reasons to go into additional colors
Appa, Steadfast Guardian - $7.70* to $27.50 *Probably the best airbending card in the set, Appa had actually already bounced back pretty hard by the time Avatar released and was valuable on day 1
Thats not to say the game lacks depth it has that easy to learn, hard to master feel, similar to the Pokmon TCG
This thing could be massive in control decks, and its sure to be a target for plenty of removal spells the moment its played
All permanents with daybound will be on that face when it is day, and will all transform to their nightbound side when it turns to night be having a player not cast any spells on their turn