Why not allow the player to decide what DJ Sona's default "mode" is?

If you read the post it specifically said your able to change the mode your in. The default is Kinetic when you start game. If you press /toggle it will change to Concussive if your currently on Kinetic. If you press /toggle again you will change to Ethereal if your currently in Concussive. And if you press /toggle again you'll change back to Kinetic.

It would be cool to change her load screen art and her base mode by choosing your icon :D

They do actually. You can decide which theme you want (Kinetic, Concussive, Ethereal) by typing in a command.