Dimir Midrange has a deep interaction suite (Counterspell, Force of Negation, Spell Snare, Thoughtseize, Fatal Push) so Trinisphere is excellent, taxing their cheap countermagic and removal to the point where it stops being efficient. The extra Vexing Bauble is a real blank check against their free-ish interaction like Force of Negation's alternate cost and punishes any zero-mana spells they might sneak in. Sowing Mycospawn's land tutoring is replaceable value against a deck that will happily Thoughtseize our ramp pieces, so we trim it for more proactive disruption. Keep the core ramp package intact since we still need to get to our top-end threats before their Psychic Frog or Orcish Bowmasters grind us out.