Singed doesnt really need nerfs, its just dark seal.
The item is already insane as 1 and when you can effectively build 2, 3 and even 4 it becomes too abusive and since you get most of the gold back as you build items and sell it theres basically no drawback into it.
For you to have an idea, without the potion passive singed gets 700 gold worth of stats for 350 when he buys a single dark seal, so he basically gains a kill worth of gold each time he builds one, this is at 0 stacks (0 extra ap) and also completely ignoring the extra healing potion passive (which also stacks with extra dark seals)
Maybe take off 5 ap from it (so its not just a staight up better dorans ring that is also cheaper even at 0 stacks) and make the potion passive unique, maybe make it sell for half of its price rather than 75%.
is the only one "abusing" the items, it's likely a champion problem, not an item problem. To be honest though, I really think
at worlds. She wasn't too oppressive, and other top laners had a chance to shine as well.