Zap upgrade ideas

Zap should be able to be upgraded once every 5 levels and each upgrade is another enemy each bolt can connect to so at level 3 it would be like a 3 stage tree diagram.

e.g.
image
reference for image: http://www.bbc.co.uk/staticarchive/8cc86f6b633c3f0b01e0301067043d2dfb74defc.gif