Hello,
Thanks for your returns ! I'll try to answer to all your questions.
So far I intend to build a final upper deck that will be removable. I'm still working on it because as Cousarmy0001 said, I have to keep balance between height and length. But I want to finish that part and see the whole look of it before making any changes.
Then I will probably had hull parts, I agree it can only add something and make the whole more impressive. About the canons this is really nice indeed, but a lot more tricky. I am not sure it will allow me to keep the hull shields (Blue arches). Of course the idea of 28 canons is thrilling and I'll think about it more than twice!
About the stern, I'm building a dormitory. I did not find any satisfying way to put hammock, so it will be bunk bed. Actually I did find pictures of bunk bed in real war vessels. On the upper deck I'll make the captain's cabin and I intend to put as much detail as I can :)
Thanks again for your very good ideas, I'm going back to it!