So, the scale of the game will be bigger than in the first game, there will be 13 fleets too.
This can give some possibilities and creation of the fleets, as mentioned in the title Battlegroups or allied detachments that we could include to our fleet.
I can think of 3 such inspiration from some TT games.
The 8ed WH40K. There is a system of detachments there. If we compose our list using specific types of units we receive benefits, We can have a few detachments, not necessary all from the same army. We do not have to hold to those restrictions but we lose the benefits if we are not.
It could be implemented in the game line a detachment of for example a 1 battleship, 0-2 battlecruiser, 1-3 cruisers and 2-4 light cruisers and if we do so we could have some benefits.
DUST 1947. In this game, armies are divided by blocs and those are divided in fractions. Fractions do not work together but there is a Block of mercenary that can fight for anyone or by themselves. I do not remember it well but it is something like that: If our list includes 80% of units from one block then we can spend extra 10% points on a hero of this block but if our list includes 80% or so units from one fraction then we have another extra 10% points on heroes. So our 100 point list could be stretch into the 120%. It is an interesting option if there could be the option of taking allies to the fleet.
Age of Sigmar Generals Handbook 2017. Here we have 2 systems here. We have 4 factions and there are divided into allegiance. We can have a list that includes units from any allegiance but if we decide to take an army of particulate allegiance, we can spend 20% of the points on allies of this allegiance (not all are allies, even in the same fraction, so we can't take a treeman to the army of dwarves) . Such allies are not part of our original allegiance so they do not benefit from allegiance special rules. Second thing are battalions that are part of our allegiance, battalions cost points and we must take certain units that this battalion includes, this gives those units some special rules that apply only to them.
This is also some idea. It could work like that: We have a fleet of Imperial Navy and we can take some AdMech or SM ships. On top of that, we decided to pay some points for "battalion" that gives all ships in it extra AA guns or something like that.
I know that such things can easily complicate the game and make balancing it more difficult but we will have 13 fleets after all. Some of them are quite small and an option of including ships from other fleets could be interesting for them. I'm interested what is developers idea for those 13 fleets in multiplayer.