What a great house - definitely keep the triangle - it is the clerestory windows that make a mid century modern house really feel gorgeous inside. Not random here. Back with ideas - but generally - on the left, a double casement, the two next to it - casements that look like they might be double - opening in a pair even though they are different rooms / post between. Do not get any muntins or cross pieces, and my advice for the right side of the house would be the same. . . big square casements that crank open . . . the entry tower glass is gorgeous . . .do you have an earth toned roof? would you like some more contemporary color suggestions?