Buildings don't have to necessarily constitute an objective themselves. For example:
1. You could have a bunch of crates stacked up to simulate a cache with a tarp slightly overhanging it, in a specific area of the forest. Boom, there's an objective. No actual buildings required.
2. Make a bunch of fake looking mortars and have a couple of fighting holes dug up around the place, and maybe some rudimentary pillboxes made of branches. No buildings, and it looks a hell of a lot more realistic.
3. Make the objective a PERSON. That's what was done at this year's Deadfall, one team's ultimate objective the entire game could be to hunt one individual as well as some other smaller tasks.
Guardians of Asgaard - KF25 -
Primaries: LCT AS VAL, Real Sword Type 56-2, VFC M4 CQB Mk2
Secondary: Latino heat, TM Glock 17