I agree with Alwyn. If I may quote myself from 2014:
"I have to say I'd really enjoy a Caesar in Gaul-style conquest of Britannia campaign. The Plautius campaign was far more knife-edge than the history books tend to say, in fact 43AD shouldn't be cited as the date of the Roman 'conquest' but rather 'invasion'. They came very close to being defeated and casualties were horrific, and Caratacus caused huge problems for the Romans. On several occasions the authorities came close to withdrawing, and for hundreds of years Britain required a disproportionate garrison, nearly a third of the Roman army at times.
A gradual campaign against ferocious foes could be every bit as fun and challenging as CiG, and late-game there could be a 'Boudica's revolt' event similar to the CiG Gallic revolt, where lots of tribes in the east rebel against you, or a Caratacus insurgency event in the West, where the Welsh Silures and Ordovices unite against Rome.
For playable factions there could be the Romans (unique leader Aulus Plautius), Iceni (unique leader Prasutagus, then later Boudica), Catuvellauni (unique leader Caratacus) and Brigantes (unique leader Queen Cartimandua). The historical battle unlocked would be the battle of Watling Street.
If the campaign stretched until Boudica's revolt, it could cover a 20-year period (6 or 12 turns per year?) or even 40 years if it includes Agricola and a map of all of the British isles, or at least all of Scotland/Caledonia.
+For anyone who really wants to get a feel for the tremendous difficulty for the Romans of subjugating Britain, the Simon Scarrow eagle series does an excellent atmospheric job (the 2 main characters are fictional Roman centurions in the middle of great historical events, rather like the HBO Rome series, but as books, and set in Claudius' reign).
The gradual conquest:
It would also give them the opportunity to include the unique design of Britannic hill forts.
Maiden 'castle':
Badbury rings:
And they released a Britannia pic:
"