The transient designer operates based on comparing 2 envelope followers operating at a floating/relative threshold to determine the attack and sustain componenets of a signal. AFAIK this is not yet possible to sample or recreate with Nebulas engine.
Likewise for the 120a/x series. These rely on synthesis (generation) of a subharmonic signal in multiple frequency ranges, which is impossible to sample with NAT (again, AFAIK unless there have been new advances here).
I don't mean to be negative, but just want to remind people that there are some limitations to what the engine can successfully sample and recreate for now. I'd love to see both of those too!
There are multiple native and DSP plugins that do those 2 jobs very well though - Check Lowender and RBass for example, and SPLs own TD (for UAD and native platforms).