I was wondering can the timbre of a marimba be changed? Maybe by changing the length of the resonators? Thanks for all of your help.
Yes, anything that changes the fundamental resonance / auto-feedback (auto like automatic door and auto like in autobiography) shall change the timbre. Timbre is basically a phenomena of which harmonics are engaged and at what strength.
When you hit an A you are actually hitting (predominantly) the A at 438Hz or 432Hz or 440Hz whatever your tuning may be, but you are also gently invoking 876Hz/864Hz/880Hz (an A one octave up) as well as notes in between and beyond. Which means that anything that affects the harmonic engagement of the instrument shall change the sound quality/flavor/taste/color.
Most obvious: you can replace the actual marimba blocks with a different material. Increasing the length of the resonators will actually change the pitch but not the timbre, a shorter cylinder makes a higher tone wave, a longer one makes a bassier tone.
Changing the shapes of the top-laying blocks (forgive my ignorance of the marimba terminology.. google says they are called Tone bars) shall change the timbre but unless the shape is fairly "pure" (rectangle or cylinder) you will experience resonance loss and the sound will be different but not as full or as robust as possible.
Anyway, your focus for changing the timbre can be: modify the tone bars in interesting ways, or modify the resonators (like with sticky tack or something that can stick inside the pipe).
If you want more insight into how harmonics make the timbre (how the auto-feedback of any instrument gives its sound its distinct flavor) I highly recommend you get a spectrogram program and run a marimba wav file or mp3 through it and watch the different harmonizing frequencies light up.
Glad to hear you are thinking creatively!