Brian's correct. It's more of what a milsim can do with the field, than what a field can do to make a game more "milsim".
Milsims at a place like PRZ out in Picton are nice. It's got a big long building for CQB, and lots of field for outdoor work. It gives a lot of variety, and a little something for (almost) everybody.
On the other hand, Brian's new piece of land is just an undeveloped chunk of forest, with a powerline running through it to break up the forest a bit. He hosted this year's Deadfall there, and the game would be considered milsim. No bunkers, no trails (except for in a small corner of the field), no nothing.
But to address the spirit of your question, if you want a place that's good for squad-based milsims, you'll probably want to look to build specific areas, that are defendable, at a crossroads in a trail, or otherwise have some sort of value.
For example, take a look at the Niagara Quartermasters field, here: http://cdn.shopify.com/s/files/1/024...r_Map.pdf?4074
You'll notice there's paths through the forest, and they go along to different bases. These bases or objectives give team commanders something to have their squads fight over. "Send squad 1 out to Fire Base Alpha, and send squad 4 out to relieve squad 3 at the Satellite Relay, and have squad 2 patrol up the paths between the Slums and Firebase Alpha to make sure no one cuts through the forest and bypasses the firebase".
In other words, give your field distinct landmarks and/or defendable positions. It will allow the commanders to easily sort out the flow of their troops, and give game organizers things to work into their game. eg the NQ field has a SAM Site, and sometimes one team will need to hold it, while the other will need to blow it. Once it's blown, airstrikes can begin to be called in.
You should also try and set things up so that there's a direct way to approach a target, and an indirect way. For example, a squad could just roll up the path to the front gates and try to shoot their way in. Or they could put in the hard work of bushwhacking or bellycrawling through the forest to try get around the target.
Lastly on this list-from-the-top-of-my-head, is variety. Pretty self explanitory. Have some open fields, have some forest, have some kill houses/little village, have some large fort-like structures, etc.
Bounding (provide cover fire and movement) through fields and forests is fun. Clearing a kill house and/or a village is fun. Defending a fortress can be fun. But these are all things that require a team effort, and it encourages squad-based play.