Dark summoning is removed in sigma - when I heard that I thought how awesome it would be to have a hidden summoning boss. Basically make steel terror a TU, die in a certain area and 24 bosses spawn!
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I would also lvoe to see set runewords! And more sets with a morph effect.
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What I really would love would be an unarmed skill tree for assassin. (If she actually hits with kicks it would be perfect)
Skill tree name: Legend of the fist* or Brawler
(*just for reference -
https://www.youtube.com/watch?v=bqfjxLCoxFo).
Basically the spells just work if you don't wield a weapon/shield. Losing 2 item slots is hard, so the skills have to make up for the loss.
Stormfist - a really fast whirlwind (+500 velocity while casting) with 100% to cast something like stormblast/ancient hand on attack (while casting)(Just imagine how that looks!)
Way of the fist - Basically like Krakenstance instead of cold spell dmg flat dmg and slows target by 20%, Area of effect attack
Zen balance - 2k HP +100k Defense +1000% total character defense (no shield = no block), +1k% attack rating, -25% to all enemy res, 30% to cast ancient hand on melee attack, 25% to cast shadow blade on striking.
Kamehameha - WDM glowing ball(s) - 100% on kill proc corspe explosion 3/2 wdm 15 yards.
Dr Dragon - Melee blink Kick that unleashes various additional effects (kick since it always hits)
first charge - gives 100 velocity, x% avoid, 100% life leech, 1k life after kill (hitting stops that charge obviously but not kamehameha)
second charge - if you hit now you instantly die but summon (like respawner) a bunch of dragon eggs that summon a friendly dragon if you destroy them.
third charge - casts a lvl 60 broadside
All melee attacks release the charges.
I am not very creative

what's very important is having at least 3 distinct spells/attacks that all can be a viable as a build. Maybe combine way of the fist with zen balance and give it a 4th attack or even a spell (patriot sentry or a trinity nova sentry)
Way of the fist - Basically like Krakenstance instead of cold spell dmg flat dmg and slows target by 20%, Area of effect attack, +2k HP +100k Defense +1000% total character defense (no shield = no block, doesn't affect the 100k def), +1k% attack rating, 30% to cast ancient hand on melee attack, 25% to cast shadow blade on striking