The issuance model is not fixed yet. It is an ongoing discussion whether or not the costs for the consensus needs to be financed by inflation that basically taxes every coin holder or if transaction fees are sufficient.
Please note that the costs for secure consensus are much lower with Casper compared to current proof of work. More details here
For this reason it is guaranteed, that the current issuance amount will not be increased and might even drop to 0. Another ongoing discussion is whether or not issues of new coins should be used to fund ongoing development. A prerequisites for this would be a DAO with a widely accepted mechanism to control spending of funds effectively. But also in this case the newly issued coins would not increase.
