It's a common problem. Blizzard announces a new patch with tons of new changes and says, "The PTR is up, go test stuff!". You download the PTR, smile wide and spirits high, only to find yourself waiting in 600+ second queues because there is either no one else testing (common on most days past the initial go live of the PTR) or everyone is wanting to test the same thing. This has been a real problem since the inception of the PTR and greatly limits the amount of actual, real world testing that can be done. Sometimes this isn't really a big deal. It's just a new hero and a few balance changes, and inconsistencies can be fixed relatively easily once the changes are on live. However, for patches like the 2018 Game Update, PTR is not only important, it's essential for allowing players to test, and give constructive feedback on the new systems in as close to a live environment as possible. Sadly, this just isn't the case right now. So how can we fix it?
Incentivize. Yes, it's a bit of a buzz word, but it turns out that if you give people a bonus or a good reason to play something, they'll play it. Incentivization is something that our current PTR system is lacking. Sure, in a perfect world you wouldn't have to incentivize PTR play, and people would jump in and switch most of their playing time to it the week it's up so that they could provide valuable, constructive feedback to the Heroes team. To the people who actually do that, we salute you, you're awesome. The general Heroes population though will only play a few PTR games, if any at all, during the PTR testing phase.
The first thing I would suggest doing is a Live/PTR account sync. Now, we'll ignore any potential engineering issues that might come with that (because I'm sure there are a few), and look to what it could achieve. To start, you wouldn't want to sync absolutely everything, but syncing Hero levels and xp gain would be a tremendous start. This would allow players a real sense of progression as they played their PTR games, allowing them to progress their Heroes (and on the side earn loot boxes and potentially gold in the live game), and cut down on the "sunk cost" feeling of PTR testing being a time waste. I feel this would be a good compromise, as, from Blizzard's perspective the player is playing the game as they usually would, only now they're also providing valuable data and feedback that they wouldn't have otherwise.
Another potential option (in case syncing turns out to be really hard) would be to use the quest system. The PTR could offer repeatable quests for games played (potentially even customized depending on what needed testing the most) that then translate into gold, shard, or gem rewards on the live servers. You could even offer a scaling quest, much like the current Dragon event that is going on, that offers tiers of rewards based on how many games you play.
Whatever the method, I think that a more consistent, populated PTR testing experience should be on Blizzard's radar. This only becomes more apparent when it is extremely hard to test large, game altering changes to allow for informed feedback. The fact that Blizzard scheduled a three week PTR for the 2018 Game Update shows that they are wanting that informed feedback. Now they just need the mechanisms that will incentivize the population for it.