You have the shape change ability scale with level. This would let the Yuan-ti take on more powerful snake forms as they advance and still be a lot less powerful than an actual druid because of the limited amount of serpent stat blocks.

A Yuan-Ti Pureblood that can use the shape changer ability to morph into other humanoids.

When a Yuan-ti shapeshifts, it loses the magic resistance until higher level.

• Level 1   - Flying Snake, Poisonous Snake
• Level 4   - Giant Poisonous Snake, Constrictor
• Level 8   - Giant Constrictor, Swarm Of Snakes, Fire Snake
• Level 14 - Humanoid Shape Change, Giant Coral Snake
• Level 18 - Humanoid Shape Change with Magic Resistance
• Level 20 – Shapeshift with Magic Resistance