I don't agree there is anything fundamentally 'entitled' about expecting to receive some inheritance from a loving parent, although I think there are good points made above about not relying on this financially. But to me the main thing that comes across in replies here is how hurtful it is emotionally if children are left out of a will, for whatever reason, careless or otherwise. And this is far more likely to happen with the complications of merging families.
But the OP is in an impossible position; if you don't mention it, you may regret that later, but if you do, you risk destroying your existing relationship with your father and stepmother-to-be who could resent what they perceive as your greed and unwarranted interference. In a similar situation, I got sucked into a corrosive and destructive debate after my father invited me to the solicitor's appointment to plan his new will and then didn't act on the advice. It's taken years to patch up the relationships subsequently and that was only because I backed off and suppressed my true feelings of rejection at his decisions. So be very very careful, even if you think you're in a family where you can talk to each other about anything.
If you feel you have to say something, my suggestion would be to broach the subject in an abstract way, alone with your father. Definitely not by email. In the context of saying that you're happy for him, mention that family rifts can occur with these extended stepfamilies over money issues and inheritance and that you hope he will always be careful to be sensitive of that. Then leave it at that. Do not get into a discussion of what might be 'fair' or what he plans to do, even if he starts to offer suggestions. And then try to let it go, to preserve your own sanity (you have no control over his choices and less influence than the stepmother will have), and focus on all the positives for the family - of which thankfully it sounds like there are many.