Mechanics.

[Arcanis222]

List Of Mechanics We'd Like To See Implemented Next:
Persist. Kitchen Finks
Kicker. Degavolver
Retrace. Raven's Crime
Any Others?

[GandoTheBard]

Buyback, Flashback, Suspend, Cascade, Bushido, Rampage, Dredge...that is all off the top of my head.

Plus mechanics for tap/untap like Icy Manipulator, Tradewind Riders, any thing else that uses "tap ... creature". (Convoke!)

Oh and cards that target the graveyard (Feldon's Cane, Tormod's Crypt, Scrabbling Claws etc)

[Arcanis222]

Agree with the graveyard, I'd Love Unearth and that zombie knight legend who's name i never remember...
Also Cascade would be awesome, But how plausible is that?

[apthaven]

It would be interesting to also see Soulshift too. And all the bunch of Arcane and Spirit cards that rely on them.

Lots of non-Kamigawa spirit monsters are itching to benefit on them too.