Off the top of my head the first major don't I can think of, and see it being done recurrently, is easy:
Avoid making plr even MORE linear than it already is by nature!
For fuck's sake you're pushing a cart along a predetermined and unchangeable path, going head-on against the enemy team. The last thing you need is for the MAP to force you to follow said path. If there is no way to circumvent the cart path, or insufficient ways to reach the enemy base without colliding with their entire team or being forced through a choke point, your entire map, as beautiful (or not) as it may be, becomes a battle of attrition. Just bodies constantly thrown into the meat grinder till one side manages to grind away to the end... This is not fun. The gameplay becomes a chore.
There are generally 2 ways to go with plr:
- Have a main linear path for each team going in opposite directions alongside each other and a couple of side routes that lead behind the enemy (careful not to make it too easy to spawn camp), which is how the first 2 "rounds" usually go.
- "Open", "side-by-side" map, where both teams race to the same point, in the same direction. Usually the last stage of a plr map.
Whichever you pick, it's important that both team's carts stay close to each other, enough so you can meet their team (or cart)at any time while pushing (or heading directly to) your own.
I know I started by saying "don't make it a meat grinder!" and I'm now seemingly contradicting myself by saying "do make them inear!", but I'm not. The main path needs to be linear and force a meat-grinder. This happens to ensure that teams need to flank the enemy and use more tactical approaches than "rush now!"to avoid a meat grinder. If you place the carts far apart whichever team wins the initial "tug-o-war" at the start pretty much denies the other team a recovery by forcing them to either attack (and loose because they have a head start) or stay constantly on defense too far from their cart to try an offense. Thus you force them into defense, and thus meatgrinder ensues.
I know it seems paradoxical at first, but while the first offers the possibility of a meatgrinder, but alternatives to avoid it, the second tends to force the aforementioned battle of attrition, regardless of how many side routes you provide.
So main path, where the cart goes, should go straight into or alongside the enemy team, while providing alternatives to circumvent them.
And this is the second main don't I can think of: too much of anything is bad. Yes, there is such a thing as "too many alternatives".
Less frequently than the previously mentioned "meatgrinder mode", but equally disruptive is when people try to overcompensate linearity by including a million side routes... This is bad. Again, I know it sounds paradoxical to start by saying "You HAVE to include side routes! linear is bad!" and then follow up with "...But not too many.", but there's a very simple reason for this: If your map has too many options, too many routes, people will get lost. They will get lost, feel confused, and again, stop having fun. If your map starts feeling like a labyrinth, and more people are trying to find their way to the fight than actually fighting, you have too many routes.
See, while having too few routes tends to concentrate the fight entirely in one spot and keep it from flowing, too many routes just spread the battle too thin, fights become sporadic, keeping it from flowing again... Ultimately, what you're trying to achieve is this "flow", a fine balanced tension between main path and alternatives.
A good way to visualize it is as, well, a pipeline. Picture each route as a pipe in a steampunked machine. And both teams are steam. If you only have one pipe, and steam coming from two ways, pressure just builds up in that pipe with no possible escape or release. The pipeline stagnates. If you have too many pipes for the steam you have, there is no preassure whatsoever, so, again, no flow. What you want is a main pipe to build steam, but some extra ones to offload it too. Not too many, but not too few either.
Well, hope that helped in any way...
Sorry about the huge post, I have a tendency to write huge essays...