I just thought of a couple of ideas for frozen heart to be viable
Idea 1: Reducing its constant effectiveness and improving on the effect when the moment demands it
remove the aura and change it so it could be something along the lines of when casting an ultimate you get an aura around you which reduces attack speed by X % (maybe movespeed too) obviously stronger than 15% maybe likk 50% ... could be great for them hard engage tanks like malphite etc (Scrap this i just realsied its basically current nunu ult but then again he is getting changed lol..)
idea 2: turn it into an eative item as a way of preventing true damage ? or even not block it some other way
Could be something like when activated freezes you blocking all true damage maybe giving 50% damage reduction too however youre movement speed is majorly slowed