Questions to consider: clinical specimens:

Few living donors will be able to provide grams of material, particularly not from early stage cancers (more commonly detected in Western countries), or from small tumor sites.

Potential exceptions include CRC and ovarian tumors, amongst others. Ovarian tumors may be quite large, but the bulk of the tumor may actually have spread into the omentum. Are such invasive tissues good options for your research?
Normal tissue will usually be normal-adjacent to tumor, or another disease.

For obvious reasons, the need to spare as much normal tissue as possible during surgery to remove a brain tumor means that very little normal brain tissue is available from clinical donors. Colorectal cancer surgery, on the other hand, generally includes a fairly wide margin so that the surgeon can be confident that all of the tumor has been removed, so you are more likely to be able to obtain normal tissues from this type of surgery.

Melanoma primary tissue usually goes for diagnosis, but you may be able to obtain fresh metastatic melanoma tissue, or FFPE blocks that are more than 10 years old.

If you require very large tissue samples, deceased donors may be more suitable for your work.

Another consideration is that if you are moving a diagnostic close to clinical application, it has to work on relatively small amounts of tissue or blood in order to be of practical use in the clinic.