Midnight blue (preferably with different lapel type than your black dinner jacket) would be a nice pickup for someone who wears black tie as often as you do. If you're on a 1 in 1 out plan, probably replace the seersucker as the cream jacket you already have should cover you for the warmer months. Plus you'd have two pairs of trousers (black and midnight) to wear the cream jacket with. Variety can be nice.
Yes, possibly a shawl collar for the midnight blue, but I'm undecided at the moment. I would not get matching trousers. I just can't see going all midnight blue.