alrighty, here's some brainstorming at your specific questions.
First off, I assume a battlemat. if you don't have one, pretend you do and mentally adjust what I'm suggesting to fit your game (and the fact that I don't know 4e and aren't using their terrain terms)...
We know from game play, that the fight doesn't force scrolling of the map. Unlike a videogame like Baldur's Gate, Champions of Norath, Marvel Ultimate alliance. This means the center is "between where you say the PCs and the NPCs detect the other".
So, one side starts on the left, the other side starts on the right of the battlemat. If one side knows about the other, the unwitting side is going to proceed to the center, and the watching side is going hide somewhat and wait for them to get in range, or get into the open.
So build up a jumble of "relevant" terrain around the edges, making sure there's some cover and opening on both left and right side. Basically this is the terrain that prevented either side from seeing each other initially, and what then gives the first to notice side an advantage.
I'm assuming that both parties were walking toward each other on a woodsy trail (or waiting in ambush on the trail).
Now, in the center, the "open" spot is simply more open than the rest of the map. That might mean a full clearly, a bend in the trail that's a bit wider, less brush or trees. That doesn't mean it is empty, just more sparse, which is why it makes a good attack point.
In fact, by making it more open, but not empty, you're setting up cover and hiding positions in the center, forcing people to move around them, not just advancing in straight lines toward the enemy. Imagine what I'm describing as a dungeon room with 2 exits that aren't on the same wall. Now imagine the walls are not solid, but made of trees and brush. Now imagine the room has scattered natural terrain, instead of dungeon furnishings.
So with this in mind you put "lines" of the terrain types you want, a line or arc of bad terrain is between the PCs and the "blob" of good terrain.
Now bear in mind, what I'm describing is pretty vague and is not the only way to look at or arrange a combat scetting. You could model the PCs coming down a narrow trail from top to bottom, and the ambushes attacking from the right side, in the trees. Your center is "the trail near the enemy", and the PCs will get attacked when they reach it.
In this case, your interesting terrain is what's off the trail. There may be a high mound on the left side of the trail that the PCs could take cover from arrows, but it's blocked off by spikey brush, so they have to go around. Basically, they're going to retreat from the center, to get to this spot. This can create a rolling battle, so be prepared for a bigger map.
If you make your center such that the fighter can move in a nearly straight line, unimpeded to the enemy, then you're not using terrain to make the fight interesting. If the fighter has to slow down, or shoot because that's faster, now the terrain has affected the battle. And that's the core point of this discussion.
In all of this, remember all you can do as GM is layout the terrain to make it interesting or challenging for the PCs. You can guess what tactics they may employ, but you must realize they could do something completely different. You'll have to be ready to fairly abjudicate that. If they want to retreat, it might be wise to "let them".