Well... I do ambient as well, and have the P12 module... I'd say that any other type of synth that has some kind of either sound or synthesis method than the P12 would be a good match... but as the P12 is a pretty good allrounder, it might overlap a bit.
I've tried to narrow my selection of synths down to just eight in number, and have used quite some time and money figuring out what worked best for me, they are:
DSI P12 - for doing VA, FM and Wavetable synthesis.
DSI P08 - for doing real analog subtractive synthesis.
DSI Poly Evolver - same as for P12, but it got some differences and another sound character.
Waldorf Blofeld - VA, FM, true Wavetable, ROMpler with own samples.
Waldorf Pulse2 - Analog Mono, powerful multimode filter on this one.
MOOG Sub37 - Analog Mono, powerful filter with character.
The major decisions on these was two things: 1. Very Deep engines with mod matrix, 2. Full MIDI specs.
The deep engines is a must, as with ambient music you want to be able to create the most weird modulations quite often. Also it's important for me that many of the synths are polyphonic.
The two monos I need because of their bass sound capabilities, as I don't find any DSI instrument capable of the type of bass sound that I want to make, especialy in boming sequenced Berlin School like sequences.
I like all the above synths... they are also relatively modern, meaning no maintenance of old quirky stuff... almost all have an analog part in their signal chain, as I'm a natural analog guy... only exeption is the Blofeld, which will go as soon as some company makes a new sample/wavetable oscillator synth with analog filters on their voices.
I still have space for two synths in my setup, but nothing really impress me at the moment exept maybe Hypersynth Xenophone... otherwise I'm waiting patiently for DSI's next reveal
Hope that was of some help...