Although trees will naturally act as a good buffer to help ID the different parts of the pedestrian / cycling / streetcar lanes (not to mention bring much-needed shade), I think it's possible they could make things worse. Reduced line of sight for peds, cyclists, and streetcar drivers...may equal more opportunities for collisions.
The area's fantastic, and the improvements made are simply astounding. However it's definitely unfortunate that QQW is a bit of a victim of its own success. Last summer what I saw was insanity, both good and bad. Throngs of happy people, selfie-sticks galore, but lots of close calls and rude cyclists yelling at families. Haven't been down there on foot or bike in the last few months, but I think things will only get worse. And unfortunately collisions will increase in frequency and severity (be it cycle vs ped, streetcar vs car, ped vs streetcar...or any combination).