No base armor on Heimerdinger forces him to mid lane.

tl;dr Heimerdinger can not afford to autoattack melee champions top lane anymore as he can't get armor and will end up getting blasted by minions)

Preface this to say: Heimerdinger's turrets autoattacks draw minion aggro (This is the only instance in the game where a champions abiliities will draw minion aggro, case could be made that azir's soldiers Auto Attacks which are an extension of himself does but other that that, that's it.)

Previously, in top lane you would drop turrets and then auto attack harass + turrets. Once you attack them one time your turrets will auto and YOU draw minion aggro (as opposed to your turrets). The minion aggro range is way further than standard (You can be at your turret and if your turrets attack them by their turret minions will want to come towards you, this may be exaggerated but you get the point that it's hell of a distance away). Even with the runes for armor, you still would take a ton of health without abusing vision constantly (and as soon as you left the bush they'd come for you again).

While most champions got buffs (5/10 health, 5/10 damage, 5/10 armor, 5/15 on abilities) he received 12 health a level. Google Spreadsheet - Has Preseason Stat updates for all 138 champions

Almost all enemies he would face top lane have more armor + ad (making auto harass pointless on top of minion aggro) and they gained on average 8% extra damage (on top of the 50% extra melee damage to turrets).

None of the runes actually really help his laning phase (top lane) for what he has given up and others have gained. Forcing him midlane where he has to go against assassins and people that can instaclear turrets with abilities.

Edit: Best way to fix this is give him the standard 9 armor that others have gained or change that turrets don't force him to draw aggro but the turrets instead.