When I put together a similar rack I added an Intellijel Dixie+ which has been incredible, and I'd add another if I wasn't rapidly running out of space. So, I'd suggest an analog oscillator, a Dixie+, an STO, or something in that vein. It's a radically different feel than the Castle but they can compliment each other really well.

To the effects, I haven't quite solved that myself, but it does seem to me like you have a lot already. Echophon + Erbe-Verb + Rings is pretty solid, but perhaps an Erica Synths Black Hole DSP, a 4MS Dual Looping Delay, or a Happy Nerding FX Aid?