Example I want to focus on a totally melee looter
Okay, you'd want Strength and Perception
but I have to waste points on ranged weapons just to improve my looting
Wat. You don't have to put any points into ranged perks, just buy up your Perception and buy the Lucky Looter perk. You get a little accuracy/dmg on ranged from having a higher perception, but that eagle-eyed eidetic memory is also what helps you find the best loot.
or dump points into intellect just to get better transport
Oh, so you want to be a totally melee looter
who also can build vehicles. Well yeah you're going to have to spend points on building vehicles then.
Or I want to try a passive engineer/builder relying on trader and traps
Great, in that case you'd want Intellect and probably Fortitude.
but I need to dump points into melee strength or play with just 2 rows of backpack.
Oh, so you want to be a passive engineer/builder
who can also carry a bunch of stuff. Then yeah you're going to need some Strength. Or, since you're buying up Intellect, use vehicle storage instead of inventory storage.
The A17 system is a cousin to almost every RPG system out there. Earn XP, get skill points, spend them on attributes and perks. Wouldn't surprise me if the D&D Player's Handbook was sitting on a desk at TFP. It's tried and tested, though obviously it needs to be balanced (and I'm not arguing A17 is properly balanced yet - IMO it's not).
I think specializing in an SP game is necessarily more difficult than in a co-op (not free-for-all PvE, but true co-op) game. If you're all alone, you kinda have to be able to heal yourself a bit, and craft a few things, and cook food, and shoot/hit zombies. You can kinda be stronger at a few things, but it's probably going to be difficult to completely ignore more than one attribute.
I also think that would be a difficult thing to balance out of the game, by which I mean to balance it such that SP can specialize just as much as co-op MP without breaking the game. It's baked into the survival aspect of the game, and complicated by the fact that any SP game can become an MP game in an instant. Every client is a server if it wants to be.
It would be great if we had different alternatives to reach these perks, maybe trader stocks vehicles at very high prices or maybe we can gather difficult resources + high dukes so he can build 'if' he has a working bench. Or maybe more fleshed out quests for certain rewards.
I think those are interesting ideas that could help the SP player specialize more deeply. Note that extra inventory space can be had through armor mods and there are various ways to increase attributes and (temporarily) gain perks without buying them through points.