Heroes 5.5 WarGame Edition - Tactical Edition.

I was thinking about this, but it would look strange if strategies involved a player intentionally delaying the capture of a town to get extra buildings for free, which would accumulate in the town every day.

There are such strategies against AI castles, but as an option, you could do the following:
Option 1 - slower construction (1 building in 2-3 days)
Option 2 - automatic construction of all buildings at a level (in one row) on the first day of the week, with a bonus for them (in the first week - level 1, in the second week - all buildings at level 3, in the third week - level 5...)
You won't have to wait long, as the games are relatively fast, and at the same time, the number of troops in neutral castles will increase.

P.S. When are you planning to release a new patch (release) for WGE duels?