I would have liked any number of things, so many suggestions have stood out. I would have liked Engagement moved to level 40, learned at the same time as Displacement so they're really seen as two sides to one coin from the start. In its place they could put anything to make RDM more interesting.
Support Wise:
Erase on a 90 sec cd at like 400 potency, usable on self or others would have filled out survival niche, while providing support.
Phalanx as a targetable Vengeance like buff to help mitigate damage and deal damage back (if it was on charges this would be pretty good, so you could use it optimally without just using it off CD for the damage.)
Single Target Damage Rotation wise:
Something that consumes or Converts procs, so you can just eat the proc before overwriting it with Verholy/Verflare if you couldn't manipulate beforehand without overcap.
AOE Damage Roation:
Something that changes VAII/VTII into our AOE version of Verholy/Verflare with VerWater/VerBlizzard.
Really anything. RDM feels allot less in control now that apoc is gone, as the caster with the most OGCD windows it was very easy to incorporate into battle on the fly.
Right now though, all it really "needs" is potency buffs and Magical Embolden. RDMs should not have to avoid BLM/SMN or suffer a large RDPS loss.
