What carrier can I activate my iphone on?

I have a Verizon Iphone 5c. This device has NOT been reported stolen or lost and has a clean ESN in that respect. It is however currently activated/connected to an old verizon account. Are there any Verizon MVNO's it could be used on? Any carriers it could be flashed too? Any way to unlock the sim card slot and use on a GSM carrier?

It is already unlocked because it has been activated on Verizon. There are no Verizon MVNOs it can be used on, though. Flashing is a hassle, so I'd just get a Nano SIM for any GSM carrier.