Null shards - Like a null stone only it prevents x amount of spells and is then used up, maybe you could toggle it off and on if you made it only absorb the 1 spell.
Spell Reflector - An item that will allow me to reflect the next spell.
Could reflect a % of the damage or the whole thing, maybe even reflect it to a random enemy hero in the area. If it's a 1 shot item that you can activate or a expencive item that will allow you it more often i'm not sure.
Net - A 1 use item that you can throw over a small is area and pin anything in place for a few seconds, fairly small range/aoe but not a stun just a root. Could be used to avoid melee types trying to ruin your day. Penetration items - items that increase armour penitration and spell penetration, effectively lowering the armour/spell resistance for your attacks/spells.
Land mines - Simple cheap things you can try to lure people over or use to protect towers. Hurts heroes only, or not depending on balance, maybe you could throw them at peoples feet to add a bit more damage to your dump aswell. Also lets you see know where stealthers are if they walk over them. You could get more damaging ones that are more expensive or a mine layer that costs alot but but lets you put one down every X secs.
As for prices, damage and stats i don't know enough about the game to know what would be balanced or not so i won't embarrass myself by guessing, i will leave that to the devs or anyone who might have a good idea on how they could be implemented without messing anything up.