Is there a papaya substitute in salads?
The "key property" of the papaya and the reason why it is used is its texture: crunchyness. I think you can use anything else that is crunchy and has a quite neutral taste, like cucumber peel, cabbage, kohlrabi, carrots, young lotus stalks (like the ones in this receipe, not these), or unripe mango. I had them in salads similar to the green papaya salad and found that these ingedients are quite neutral in taste.

After all, replacing the papaya makes the salad to not be a green papaya salad anymore. ;) You could try other gỏi -- vietnamese salad -- receipes.