Personally I think they'd skip letter I, as well as O, Q, S, X, and Z. Those letters are typically not used in drawing view titles (i.e. View A or Section B-B), and are also not used in drawing revision letters. They're either too similar to numbers, each other, or are reserved for other uses (X).
I can go with that.
Then, in that case, I'm going with...
25th Century
Enterprise-G
Enterprise-H
Enterprise with a New Registry Number
Enterprise with a Second New Registry Number
Enterprise with a Third New Registry Number
26th Century
Enterprise-J
The 24th Century had five Enterprises (B, C, D, E, and F) of varying lengths of service, and at least one 20-year break (between the C and D). So, I'll say the 25th Century also had five Enterprises. Also, of different service lengths and with another extended break in there somewhere.
In my mind: the 23rd Century had four Enterprises. I count the 1701 twice, since the TMP Enterprise is "an almost totally new Enterprise" to quote Decker. Then the Enterprise-A. And the first seven years of the Enterprise-B.
I'm not going to count the Enterprise-F as 25th Century since it was decommissioned in 2401. It was barely there.
I'll call the Enterprise-J the first new Enterprise of the 26th Century, and it doesn't matter when it was commissioned. In my mind, there was a huge gap between the Enterprise-J and the Enterprise before it. In fact, given the scale of the Enterprise-J and how different it is, it's the most radical departure from its predecessors ever.
I think the Enterprise-J is what the Enterprise-D
wanted to be: a city in space, except on a starship. And this one's much larger. It wouldn't surprise me if the Enterprise-J had its own Mayor, who would be separate from the Captain.
EDITED TO ADD: And because so much would've gone in making the Enterprise-J, and given how large it is, in my head-canon I think it served MUCH longer than any other Enterprise up to that point. Possibly 50 to 100 years.