If you're up for messing about with some tunings outside of 12-tone equal temperament, you could try using
Tobybear MicroTuner with a .tun file (a file that basically re-tunes all your midi notes) -- a link to a big set of them
here, including several variations on the pythagorean scale, which as Bartekko said would be a good place to start -- alongside your VSTi of choice. It isn't something I've tried (although I want to - microtonality looks like a fun rabbit hole) but from the forum posts that popped up in results when I was trying to dig this stuff up just now, it's a pretty clunky setup that might take a bit of work to get running smoothly. Which from your previous posts sounds like more trouble than you're after, but it's there if you want it

Alternatively tune the individual notes by hand, or see if you can send different-pitched notes to different channels, and then run each channel through a pitch-altering plugin or something? -- there are lots of pitch charts for different tunings floating around Google, in cents, hertz, whatever.
Anyway this topic was about instruments not tunings, so sorry for derailing