Development files for Yoshi's Island reveal that Yoshi was originally meant to have a human voice. Lines for transforming into his various forms, including unused ones like Tree Yoshi, have been found.

But properly-executed to play to Sonic's strengths (finding new paths, finishing a stage quickly, taking down a boss before it can set a Zone on fire, etc), this would be an incredible incentive for players to WANT to replay the game, and naturally see the magic in Sonic's replay-driven game design.