Low, slack, with lotsa' standover. 67* head angle with a Pike at 140mm. 16.5" seatstays. Keep the wheelbase short-ish. Spec with a 20mm axle fork - don't screw around with QR on a bike meant to do it all. Big tire room - that is one a lot of companies really screw up on. They make a bike with great geometry and sizing, then give you crap all for fat tire room in back.
Material? That depends on what sort of price point you're looking to hit.
Cheaper alu frame with nicer spec? Ti frame with lesser parts, or the same parts but more money? Steel - light or heavy tubeset? Give you the same issue as with alu or ti.