This is where I make the biggest modification to Shadowrun. The original skill system doesn't have a lot of depth in terms of options - and while it gave players the option to take concentrations and specialization, the rules around it made it very discouraged.
In standard rules:
1) You can only take one concentration and one specialization for a given skill. Period.
2) You can only take any given skill once, ever.
3) You take a -1 to -3 to all other uses of the primary skill for anything outside of the specialization.
This was a huge blow to character customization, because there's very little reason to
not just put points into the primary over-skill. Even SR's example-characters from the books don't take specializations except where required.
It also makes every character with a given skill identical in knowledge/ability.
Shadowrun already has stupid huge dice pools.... so here's a variation that I got from one of my first ST's, and my personal favorite.
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House Rules!
1) You can take as many concentrations or specializations under a given skill as you want, and you can get as stupidly specialized as you want.
ST's call as to what constitutes a concentration vs a specialization.
2) You can spend points on the umbrella skill (general skills), concentrations or specializations (insanely specific).
3) You can put points into just the specialization or concentration without spending points on the umbrella skill.
Caution: If you don't have points invested in the umbrella and/or concentration, you will take massive penalties (Ranging from -5 to you Can't Roll) when trying to roll on something outside your specialization/concentration.
TL;DR if you try to game the system, the ST will punish you in hilarious ways that will live in party infamy.
At Character Creation:- A standard umbrella skill is 1 point spent = +1 skill point.
- 1 point spent on a concentration = +2 skill points to that concentration.
- 1 point spent on a specialization = +3 skill points to that specialization.
So, if you decide to take ((G)Firearms -> (C) Pistols ->) (S) Walther P-32 pistol, you can spend two skill points on the Walther and get a +6 when using that gun.
But, if you've spent no points on (G)Firearms or (C) Pistols, your character has no clue how to fire any other gun. If you try, hilarity will result.
If you've invested a point in firearms, you at least have general idea of how all guns function. That point counts as an additional dice toward all firearm rolls.
So someone who decided to spend points like this:
Firearms 1
(C)Pistols 1 (2)
(S) Walther 2 (6)
You'd only have the one point for Firearms, but with Pistols, you have +2 and when using the Walther, you would have +6 dice.
In game terms, you have a vague idea guns work on principle, decent idea of how to use Pistols, and damned good with that little Walther hold out pistol.
On the flip side, if you have no specializations or concentrations, rolls for specific technical knowledge/elaborate tricks will have much higher difficulty.
Example: If you take general Biology, it represents general knowledge of Biology - stuff you'd get out of a textbook, good general stuff to know.
But if you're wanting to use that on a roll to look at gene-sequence analysis data from a hit on a Aztechnology biolab to figure out what they were trying to engineer, you will have Clue Zero what the data is for. I don't care how many successes you get.
As for skills themselves, the sky is the literal limit. We have people who've taken (G)Film Production -> (C)Simsense -> (S) B Movies as a specialty, (S) Painting and Decals for Cars.