I have some humble suggestions
1. A Mutron-III emulation ( used by Bootsy Collins and Stevie Wonder)....the best freeware so far is to my opinion "northpole" from prosoniq, but this is a wild beast, difficult to tame.
2. A good Vocoder with at least 128 bands, the best I have is the CM-Vocoder with 64 bands
3. An additive Synth with a good resynthesis
