I love Malzahar, and with further buffs, deathfire touch has made Malzahar's Space AIDS go from:
Release: 1.1 scaling
Damage type adjustment: 1.2 scaling
Current: 1.3 scaling
I just hope this doesn't result in further nerfs to the champion itself. His w currently feels very negligible, getting off maybe 2 ticks of the damage since they can simply walk out without ult support, resulting in a possible 8% of max health after usual ADC MR, and possible 3% on the usual tank, excluding magic pen. There's a good chunk of damage if you ult, but still that tank buster feel is a little gone since tanks these days have considerable damage.
Malzahar definitely fits within the realm of immobile mages, and I'm sure there have been plenty of things being tossed around on how you're going to rework him.
QSS on ADC's has become even more reliable with the new Merc changes. I'm not sure why he is seen as too large of a threat apart from his lane bully demeanor? He's generally good at 1v1's because of his kit, but most people who play against him know hard CC or QSS really counter his kit as a whole.