Open space in the desert *ponders*
Well, that's everywhere - and it does depend on the level of the "Shooters" hehe
For lower level (and to make traveling eazy) my first suggestion would be at the Karavan alter/port at Pyr.
It's very easy to get to - it's safe and it would be easy to set up for spectators or on-lookers.
It's actually pretty roomy there - with a little view of the trenches, etc.
Also there aren't a lot of mobs in that area, so you won't have herby clutter
Another intresting spot I think would work (although many may think it odd) is the isle or penisula in Dyron.
Almost everyone has access to Dryon, if not it's easily obtained as well - this is a suggestion for slightly higher players.
I can see the audience parted in the two bodies of water to watch the competition.
The competitors can line up along either side of the strip of land - maybe facing their opponents, etc.
The only issue here would be a couple of harvesters - but generally no mobs come onto the land.
The only time you have to worry about agro is when someone has gotten the attention of a far-off kipucka and has run toward and odd goal intead of the water a panic
Thesos - two spots.
The first one is right at the TP/Alter.
There is always an agro-awareness program here, but it's not too bad, plus if there are a lot of people at this event/competition a varnix or ocyx shouldn't be a problem.
The second spot is actually to the right of the market when you're coming from the TP - this is kind of like a walk-way out of the town.
This would be an interesting spot and as far as I've seen it's not an agro area.
I haven't spent a lot of time in Thesos - but the second spot seems like the best option for this town.
Now, for much higher level players you could always do something in PR.
But, of course this would mean everyone attending would need the TPs to the area you're considering.
There are actually a few open spots that would be perfect for it, but probly more difficult to make sure everyone can be there.
I do agree with the desert being the best option for this type of competition.
You can deffinately see easier there - wide open and line of sight is clearer.
I hope some of this helps
Have a wonderful day!