Personally, I would like to have more clear boundaries for the play area. Not only to prevent people from getting lost (which is very difficult where we play), but also to have an idea of where I need to be for a scenario. I get very confused when we are playing "Go there, do this" type scenarios. I don't know where "there" is, and sometimes I don't understand what the "do this" is either (read my post on last friday's game feedback).
I know it's not like we can just string police tape around the entire parameter or anything, but we can mark boundaries with flags or even paint on trees (who owns this property again?).
Having fortified positions is a wonderful idea, I also think that respawn areas (once we have more than one for extended play) need to be identified a lot more clearer than we have so far.
As a side note, I think we need to make clear our respawn times/rules. My favorite for longer games is: You can respawn when there are a certain number of people in the dead/respawn point (for example, once 3 or 5 people die, then they all respawn at the same time... when the last person to die of that number gets to the spawn point and there are that many people) and/or time limit. Time limit can be used on it's own, or in conjunction with the above method. If you are dead for 5 min (or what ever time period is agreed upon) at the spawn point, and the number needed to respawn hasn't died yet, then you can respawn by yourself when that time limit is up (but obviously this will mean that if there are other dead people that died after you, they will have to wait for either their time to respawn or wait for more people to die).
I'll add more if I can think of other ideas for the actual Field.