I am new to Retribution myself, though I played a lot in MKII with Cygnar, I'm just returning to the game.
If you haven't already been introduced, I highly recommend having a look at Warmachine University
Based on what you have listed, I understand a unit of Battlemages are a common auto-include with Adeptis Rahn. Also of interest is anything with an arc node, Sphinx is fun, though he can't come in theme, Discordia is a great option with him as well. Either jack can also run with Helynna since she can pretty much make good use of any jack.
If looking for a solo, or warcaster attachment, Ret has some great ones.
Sylys - free upkeep makes polarity field free so you can TK
Fane Knight Guardian keeps the caster alive
Eiryss is a safe bet as a toolbox, but the CA
version is pretty costly