Explaining why i'm taking Remnant on Karma mid as first item
I can't tell you how many times I get my name pinged by my allies when I get remnant with Karma mid. So here are my personal reasons for getting remnant :
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It's very easy to build. So when you're behind, you can always at least buy an ingredient for the item.
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The item is self paying
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is a must on Karma, so the extra 50% mana regeneration from Remnant is like an extra 10 AP -
gives 10 CDR and 20 AP and a gold regeneration per 10 sec for 850 gold which is IMO so much better than 
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You get 4 wards and can replace your yellow trinket for oracle
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Once full build, you can sell this item and get

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Gives 200 HP which is SOO GOOD against assassin mages. Goes well with
and
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Karma is one of the rare mid laner that can afford to buy Remnant without being too much penalized. Riot has made normal mage champion rush for mana items :
. The problem is how expansive those items are (they all cost 3k gold ++). Once you get behind, you're basically way too vulnerable. Those items offers almost no defensive stats.
Also, mages now wants raw mana instead of mana regeneration since that stat is mostly for enchanters (they don't offer enough offensiveness) I don't see any mid laner get mana regeneration items other than doran's ring.