Technically, I suppose we could.
I would question, however, how much sense it would make.
Let's assume Weapon Focus as it currently is becomes a general talent that everyone can take. +6 Accuracy at the cost 1 talent point would already be a pretty good deal; at the cost of 1 proficiency, it is a total no-brainer. Everyone who needs to hit their enemies (e.g. every non-support character who doesn't rely on spells to cause damage) would take it; not doing so would be willfully gimping oneself, which most players won't do (only veterans in search of a challenge via self-imposed rules would, in fact.)
At that point, we have two scenarios:
Everything stays the same (i.e. monsters are not rebalanced around the assumptions that most characters would have Weapon Focus.) This preserves the essence of the Weapon Focus talent, which is to give you an edge in combat versus the "normality" (which is not having a +6 Accuracy bonus on every attack.) However, everybody and their neighbor would take Weapon Focus because it's up for grabs at what amounts to no cost, and gives a very good advantage. Suddenly the game is too easy—and certainly easier than intended by the developer at any rate.
Everything is rebalanced around the assumption that most characters have Weapon Focus in order to retain the challenge. This makes Weapon Focus a de facto must pick or you would be factually gimping yourself, since monsters' stats are such that you are expected to have it. Weapon Focus goes from being a talent you choose to gain an advantage to a talent you must pick to not be at a disadvantage.
I'd say both scenarios are equally undesirable. Josh mentioned in one of the Q&A streams that for Deadfire they tried to limit the amount of things that could increase Accuracy specifically because it is such an important stat. Everyone would jump at the opportunity to increase their Accuracy, and it becomes harder for designers to balance the game and provide a challenge without being unfair to players.
Now, let's assume you'd still give Barbarians an exclusive talent called Superior Weapon Focus that gives, say, +8 Accuracy instead. I see at least two problems:
The game would still need to be balanced around the assumption that most characters would get a +6 Accuracy bonus, so my point about Weapon Focus becoming a must to avoid a disadvantage still stands.
Barbarians would have a ridiculously high +14 Accuracy bonus from taking both, which would make them far superior to most other melee classes, if not all. The solution would be not to make the two stack; Barbarians would still get a +8 Accuracy for 1 talent point deal, which is a bit too good. Perhaps we could make Weapon Focus +4 and Superior Weapon Focus +6, mutually exclusive, to ameliorate the issue—but it would be too much work for too little benefit, imo, as the previous point would still be a problem.
I definitely advocate for Weapon Focus to stay where it is now. It's OK as a Barbarian talent.
Weapon Styles can become general if Fighters get something unique in their talent tree in exchange. Similarly, Bull's Will, Snake's Reflexes, and Bear's Fortitude should probably be general and replaced by something different in the classes that get them; and by all means, let's bring back the elemental talents for the general talent pool—they were flavorful and interesting, and it's a pity not to have them in Deadfire.
How about a possible minimal solution for the main concern?
Weapon focus should be definitely restricted to one or few selected class, otherwise the game needs to be rebalanced assuming that almost everyone has it.
Let's make weapon styles available to every class, since casters lack passives, and many players are eager to have the option to select them. Since fighters lose their unique abilities, they should be compensated.
While I agree that it would be best to give fighters cool actives, the simplest form of compensation is sharing weapon focus with them.
To my opinion, this solution diverges the least from the original intentions of the devs. Further refinement can be proposed during later iterations, of course.