It could feasibly work similarly to the Survival scenarios that we already have, but without the capital shipyard. Instead, you start with a predetermined fleet(based on what ships were used in the battle the scenario takes place in), and every "wave" you get a certain amount of reinforcements.
Let's say we recreate the Battle of Coruscant.(note, this is just as an example, and not necessarily lore accurate)
Let's say you choose to defend as the Empire.
You start off with:
2 ISDs
1 ISDll
2 Victory lls
2 Escort Carriers
and 3 Lancers/IPVs
Let's say you get a Victory ll and an Escort Carrier every other wave as reinforcements. Completing wave 3 rewards you with an ISD.
Now there would be 5 "waves" to this scenario:
Wave 1: 4 MC80 Liberties, 3 Nebulon-Bs, and 2 CR90s
Not overly tough, but if you waste any fighters/bombers in this wave, it'll make later waves tougher.
Wave 2: 4 MC80 Liberties, 3 Nebulon-Bs, 2 Nebulon B-2s, 2 Assault Frigates, 4 Y-Wing Squadrons, 2 CR90s
Same fleet, but with a few bombers and light frigates mixed in.
Wave 3: 3 MC80-Bs, 1 MC80 Liberty 2 Assault Frigates, 2 Dreadnaughts, 6 Y-Wing Squadrons, 2 Quasar-Fire carriers.
Wave 3 turns the heat up a bit by sending in some actual capital ships and frigates. This wave also sort of tests to see how well you managed the fighters you had started with.
Wave 4: 4 MC80-Bs, 4 Dreadnaughts, 3 Nebulon-Bs, 3 Quasar-Fire Carriers, 2 Assault Frigates.
This wave aims to soften up your defending fleet for the final wave, hoping you lose an ISD or two.
Wave 5: Home One(Ackbar), 4 MC80-Bs, 4 Dreadanughts, and 3 Quasar-Fire Carriers.
Probably either too easy or too hard, but it's an example. Either way, it could work out similarly to this.