Best way to make samples that never existed

[Auditory]

I couldn't find anything on this topic, so here goes.

What's the best way to create instrument samples that never existed?
For example, filling too-large gaps between sampled intervals in freely available public domain sample libraries, or making a sample sound like it was hit harder, and so on.

I was able to find potential solutions for percussion - there was Superior Dummer 3's tuning algorithm, but their license doesn't seem to permit it to be used for editing samples. Correct me if I'm wrong. Maybe I should ask them myself, that clause is probably there so you don't re-sell THEIR samples. There was also Waves Torque, which only seems to be for drums - unfortunately it doesn't feature a 'cymbal' mode like Superior Drummer 3.

It's possible to edit the tuning of say, a timpani sample, without the typical problems with just flat out pitch shifting. I also considered layering a faint crossfaded sample that's 1 octave higher, with the attack taken out, to sort-of imitate what happens when you hit the instrument harder, but I haven't experimented fully with that yet - I like to experiment with this sort of thing even if the results aren't 100% realistic.

As far as I'm aware, other tuning solutions are made for, well, mostly correcting vocals. Am I wrong?