There are a couple of sources to gain magicka cost reduction. The main source is obviously, light armor passives (21% 7/7) which is why it's so prelevant among people who wishes to play optimal.
However there is a problem with this for Stamina based builds. The only way for a stamina build to get magicka reduction is to either take two pieces of light armor over a full set of medium armor and then get the 20+ magicka reduction enchants or switch out their crafted gear set with seducer, however, this reduces damage because hundings rage synergize better with a bow / dw build.
So you're left with incredibly expensive class abilities, which really is not a good idea as you need whatever magicka you have for survivability. As a NB it's really hard to escape a DK when your main escape abilities are so expensive. When I encounter DK's, most of the time I'm out of magicka before I've even managed to get 10 meters away from them.
So what can we do to change this? Well, perhaps increase the value of the magicka cost reduction from 20 to 30? And then adjust the solfcap for cost reduction so that the light armor wearers won't be able to cast their spells for free.
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Id also like to see a new accessory slot: Cloak.