From a business perspective I will be surprised if we see an ISD before wave 3. Reasons being that 1) its the largest and most powerful ship in the Imperial lineup (excluding SSD's), so from a points perspective, if you own an ISD, you wont be able to field much else - which will dramatically reduce the future number of other (Imperial) ships sold to players once its released - cause lets face it, just based on looks if you can field an ISD, why would you want anything else?
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2) If you have such a powerful unit on the table you need to be able to counter it - this means a wide variety of ships needed for the rebel player to field, both large ones (like the MC80), and smaller options that can work well together, this I imagine will be wave 1 (with the assault frigate) and wave 2.
3) FFG knows that we are hanging out for it - by keeping it as a future "carrot" people are more likley to buy the other expansions to tide themselves over in the meantime when it eventually comes out
At worst - we might see everything Imperial that is not an ISD before it finally hits the table, but I dont think FFG would be that mean