After a lot of consideration and testing on normal difficulty, I come upon the conclusion that the ability tree at its current state is very limited. I hope they make changes to it to allow some variety.
From Lv 1 - 15 you only got access to 7 ability picks.
Assuming you finish the game at level 15 - here's what I recommend you should pick, no matter what your build is.
1. +10 Rifle & Shotguns: Rifle has a reliable hand-placed loot. I don't wanna spoil too much, but even the first weapon reward in town gives you a rifle with no skill check. Yeah, no ammo, but it's a Rifle. Up to you if you want to treat it like a pre-requisite to Monster Lore or make it your primary weapon.
2. Monster Lore: HP & Weapon Awareness: This pays off immediately, allowing you to pick weaker targets and judge if you should burst fire or single fire. Seeing 6 slavers is scary but knowing they have 49 HP each makes it much less so.
3. Witcher: Double Damage vs Monsters/Animals. Dear diplomats, I recommend you take Witcher Diplomacy so your Pistol can contribute against non-negotiable monsters/animals.
4. 20% Knockdown Resistance: Getting knocked down sucks. You deal no damage and everyone is guaranteed to hit you.
5. 10% DR / 10% DT: This ability is pathetic, but you only take this to unlock the next one.
6. Think like the Enemy: +DT when you suffer 4 or more damage. This matters a lot in the late game when you start facing a lot of ranged enemies.
7. Free pick.
Scoped Rifle user may want to take the 50% critical chance bonus as the 4th pick, delaying Think Like the Enemy until Lv 15 - it's your choice, offense/defense. Just be aware Oaken Skin (+10% DR/DT) is a
terrible pick at a low level. Why? Because it scales off your armor, know that Leather Armor has DR16 /DT2 - so 10% of that is almost nothing.
As usual, the melee is viable argument talks about having access to top tier defensive perks so let's assume I want to do that.
Here's what I ended up with:
1. 20% Knockdown Resistance: You get hit more often in melee, very important.
2. Oaken Skin (+10%DR/DT): This is a sad pick at level 2 unless you smoke the elevator guard for top tier body armor. Then the extra 6% DR is better than nothing.
3. Think Like the Enemy: At last, something to make you survive longer. You still want your off-hand to carry a super-stim, though. Don't think your Level 5 HP is going to miraculously survive a sustained fire though. I hope you crit with your machete.
4. Increase 20% carry capacity: Why? Because you want the shield abilities. You need the shield to achieve invulnerability, remember?
5. Shield Block Chance increased by 50%: I hope you're happy now. More reliable DT.
6. Shield gives no debuff: Congratulations, you can now tank a lot of damage. With the DT from top end shield, you should be getting really good damage reduction compared to those cowards firing from long range.
Except:
Where is your melee ability pick?
Are you going to take a +10 Unarmed/Melee at level 15? I would assume you have it at 150 already.
At level 19 - you finally take your 2nd Melee Ability - Every spent AP is added to Dodge.
At level 24 - you finally get +10% stun chance when you punch someone.
Are you a melee specialist or a shield specialist?
Alternatively, if you just want to mess around with crafted guns, then go left of the ability tree past +10 Survival. Take the no jamming, increased handmade weapon damage. Then maybe go for defensive abilities or straight down rifle +10 to the Witcher or -1 Pistol AP if you want to use the crafted pistols.