Here's my thoughts:
I'm not surprised that the devs wanted to create separation between the bow pass and introduction of Horizon Walker, as there is some very synergistic gains to be had that of course are entirely coincidental. Bows had their attack speed nerfed. Effectiveness was stripped out of ManyShot by changing the way it functions and removing the power bonus. The amount of charges the basic ManyShot has left it as a momentary burst but now, with the HW capstone it can be used as a full fledged burst, giving approximately the same amount of extra arrows as ManyShot used to, and making up for the attack speed and power loss with half the time to replenish the bursts full charge. It is what it is, and not all bad since you can stop bursting early if you want to now.
Then there is ScatterShot. I like the look of this, but I question whether it is appropriate for a single target focused tree. It would be far more appropriate as a feat with the same pre-requisites as ManyShot. I'd take it in a heartbeat, and I'd still take ManyShot. ScatterShot first for sure because bows are starved for AoE at earlier levels.
I think the target marking mechanic will be cumbersome and ultimately self defeating. Why not just make all the bonuses apply to the first target hit, or only when AF is active. If someone really is running a single target build they will use AF and be hitting their targeted mob anyway, the extra step is just unnecessary.
And of course I'm joining in on the Archers Focus pile-on: it sucks. The need to not move for 6 seconds before it starts to stack is punishing, it should be able to stack as soon as the player becomes stationary. This would go some way to eliminate the need of an active builder attack, making the stance useful for anyone who has access to it rather than whoever randomly gets an active builder. The other problem with AF is something that other builder abilities suffer from: they are all or nothing. A player can either easily stack them to max or they find it impossible to stack them at all. The way builders need to function is to have a longer duration at lower stacks, for example Archers Focus could have 3 times the duration when stacks are at 5 or less, and 2 times the duration when stacks are at 6-10. Just an example but that would go a long way to making AF good without making it stacked to max permanently. Another thing about builders: why do they reset at shrines? They don't need to IMO.
Oh and one last thing just to prove how far I've gone to try to make AF work... If Misty Step is just a repackaged Abundant Step then it can be used to move without moving, thus not interrupting AF stacking. A little pressie there for my fellow archers
All in all it's not really a tree for me (IPS4life) but if ScatterShot remains in the tree I might get it further down the line, grudgingly, when I don't have to fork out for a 'mini expansion' just to experiment with it.
I hope my feedback isn't too snarky, I don't tend to sugar coat things.