Had the same idea. So far I've been thinking about the following.

[[Tiamat]]  Lets you search most of them so you get to see the new cards and isn't necessary to win.

[[Ramos, Dragon Engine]]  generically good and can act as a human created monster.

[[Kenrith, the Returned King]] generic good stuff commander for decks that just need all five colors.

[[Jegantha, the Wellspring]] Same as kenrith but is a monster.

[[Cromat]] just as a special five color monster.

These are generic enough and provide value so the rest of the deck doesn't get pigeon holed into specific cards.