Dimir Midrange is full of cheap interaction (Fatal Push, Spell Snare, Counterspell, Thoughtseize, Preordain all costing two or less), and Trinisphere taxing those to three mana badly disrupts their ability to hold up counterspells while developing. Kozilek's Return is excellent against Orcish Bowmasters and Psychic Frog, both of which die to it before they can grow via amass or +1/+1 counters. Vexing Bauble is cut since they have no notable free spells to punish, and the extra Sowing Mycospawns are less necessary here since we're not racing their clock. Play around Force of Negation and their heavy counterspell suite by holding a second threat when possible, and use Kozilek's Return proactively to clear their early creatures.