B.Jumblr, a pattern-controlled audio stream re-sequencer (LV2)
Plugin’s GitHub page: https://github.com/sjaehn/BJumblr
How it works:https://www.youtube.com/watch?v=B_FtuLVmG9o
Another additive synth in LV2 format, EnvelopedSine: https://github.com/ryukau/LV2Plugins
(On the GitHub page there are another LV2 synth plugins as well, i.e. SyncSawSynth, TrapezoidSynth, WaveCymbal)
XPolyMonk, polyphonic (12 voices) version of XMonk with full midi support.
There you go! 
Scour the depths of the internet for all the free LV2s we can get our hands on! 
Quick question: do we know if B.Jumblr’s UI is accessible by X11? It’s a very cool engine, but I don’t think zynthian-ui alone could handle all those controls… 
I’ve had XPolyMonk compiled and running on the zynth. I was aware that the dev has been doing lots of changes so I didn’t do much about it. It’s quite cpu intensive on my pi 4. I’ll get a recipe written.
"ye-e-e-a-a-a-o-o-u-uwww… oh-yeah-youw-yeah… yeh. "
(xpolymonk for pad, grids_zynth rhythm, and noizemak3r as a bass thumper)
Always loved formant filters, but as they’re just a bunch of appropriately set band-pass filters, you don’t need a dedicated engine: helm have a quite nice formant filter with two vowel axis, and both axis are in the mod matrix 
Thanks to mention B.Jumblr @sm7x7 . The video you linked is about J74 SliceShuffler from Fabrizzio Poce. There are a lot of similarities, but also differences. Therefore I just uploaded a video about B.Jumblr: https://www.youtube.com/watch?v=n3LrpOD8MrI
B.Jumblr has a X11 GUI. There are only 5 controls defined in the LV2 plugin that can be set via a generic GUI (incl. zynthian-ui). The pattern itself has to be set via the X11 GUI (or feel free to hack the preset .ttl). 1024 controllers for the pattern wold really be a bit too much. 