@john-cox I'm leaning more towards mono-red than UR as a shell though. Besides the obvious Ancestral Recall and Time Walk, blue doesn't bring much to the table: getting your own Force of Will, Treasure Cruise, etc. remanded is clearly terrible, and even double-Preordain for UU is not that great because you will quickly run out of blue mana to chain them together.
On the other hand a mono-red shell can fuel itself with rituals (which are great since Krak amplifies the net mana you gain from them), which I think ends up more consistent even if the individual red spells are weaker than the blue cantrips.
(Either way Gitaxian Probe is an auto-include, of course.)