Some skill lines already have something similar to this, like the Warden's Animal Companions getting stronger for each skill from that line slitted.

Honestly, what I thought you were going to say with "Specialization" was being able to select one ability line as your primary skill line which would then provide extra benefits to the skills of that line. This could be any skill line, with a set of benefits befitting that skill line. Something like this:

Specialization may be chosen at any level but costs 10 skill points. It resets on skill point reset. (Indicated by a star next to the skill line title in the list)

Class, Weapon, Vampire and Warewolf skill lines: 10% cost reduction, 10% increased skill duration, 10% increased damage/healing

Crafting: 10% chance to gain additional rare materials when crafting (tempers, kuta, etc.)

Armor: Armor of this type never needs to be repaired

Mages/Fighter's Guilds: Gain 5000 Magicka or Stamina

Thieve's Guild: Double your current Fence limit and sale prices

Dark Brotherhood: Time to enter Sneak decreased by 50% and Detection Radius decreased by 3 meters

Undaunted: The damage, healing, or resource return of synergies is increased by 5%. This is additive with all other party members specialized in Undaunted. (i.e 10 players with this specialization would increase the effect of synergies they use by 50%)

World: Increase all xp gains by 10%

Alliance Assault/Suppot: Increase all AP gains by 10%

These are just some rough ideas of what such a system could look like.