I was talking to an user, and he said he likes EC2 plugin except he wants to be able to maintain minimum number of slaves always on, so that he won't take the hit of provisioning a new slave.
EC2 plugin already allows manual provisioning of an instance, so this seems doable. It would launch new instances automatically if the current count is less than minimum. Retention strategy needs to also change so that slaves don't go away below the minimum count.
The part that I'm not sure about is if this can be done below EC2 plugin, so that the logic can be reused by other cloud-like plugins.