As you probably noticed a number of indie and not so indie sequels have done very poorly lately, selling anywhere from 10 to 30% of the original title - XCOM2, Banner's Saga 2, Legend of Grimrock 2, Blackguards 2, etc. My explanation of this phenomenon is that unless you have a AAA blockbuster with massive appeal, you don't go for a sequel because it would never sell as much as the original because the public perspective would be "it's more of the same".
Now, let's be optimistic and assume that the breakdown goes something like that (based on the reviews and impressions):
- core supporters - 25% - love it, want more
- core haters - 10% - fucking hate it, will never buy another ITS game again
- kinda liked it - 50% - liked it but ... This "but" ranges from minor to major issues
- meh - 15% - played for a couple of hours and moved on, no strong emotions, no urge to play more
So if we make AoD 2, we get the core supporters and some % of the kinda liked it camp. We'll also get some new players, probably no more than 20%. So our best case scenario is selling 3/4 of what AoD sold, worst case - less than half. Thus moving to a brand new setting with different systems but the same core design is the safest bet even though it looks like the riskiest.
...
Back in June:
Original vs sequel:
Legend of Grimrock: 936,949 vs 246,684
Blackguards: 471,616 vs 178,528
XCOM: 3,304,215 vs 823,999
Shadowrun: 723,457 vs 613,408 vs 188,034 (arguably Hong Kong was the best iteration but few people cared at this point)
The Banner Saga: 592,139 vs 43,826
Success of the first game often fools developers into thinking that they can do even better or at least as good with a second 'bigger and better' game, but it's rarely the case. The only exceptions to the rule are games that offer building, sandbox, and well-executed killing loop activities that people never seem to be tired of. Darkest Dungeon is a fucking monster but I bet if they go for a sequel it will sell less than a third of the original.