Please understand that this is just my opinion, I've only done 4 placement games (won 3) so I'm probably bronze 3-2 so I understand that my opinion means very little,
this is my view on singed (I only have rank mastery 5 on him, trying to get 6 and 7, wish me luck!)
Here's the TL:DR
Pros
Pretty good at splitpushing
decent farming under-tower meaning that his tower can usually stay for a long time
Fling is a very good ability
Poison Trail, just Poison Trail, mind games, deceptive damage, and punishment
Ultimate, definitely could use work, but raw stats are powerful (granted not really engaging and all that stuff)
Cons
Passive, I'll get to this in a second
He loses lane against, I'd say at least 90% of top laners (not that he gets crushed, but he just won't be dominating or getting a bunch of kills, it's hard for to go even, much less get ahead)
His adhesive just feels like a worthless ability, I understand the combo with the fling, but since slow doesn't feel impactful, he has to stop for a moment to use it, and his damage is from his poison trail and not from auto attacks so he can't use it to catch up and wallop on them, just lead them.
Now I guess I'll just talk through my opinion on everything (sorry if this makes little sense/has no flow/doesn't help you in the slightest)
Part of me wants them to not change Singed, I like playing him and knowing that very few other people play him decently well, adding to the "how do I play against this guy?" factor to the opponent. Changing him would be bringing him to people's attentions, for better or worse, and will make him more frequent.
However, I don't believe that a champion should buy 3 dark seals and have that be the "viable" build for early game. I understand why and it's synergy, but I feel that it's wrong/weird that a champ buys 3 of a starting item so that they do better. (again, I understand the reason and everything, just something that doesn't sit right with me). Here are what I believe is the main problem with singed.
Hidden Power
We have quite a few champs around now that have very visible and defining things, we know that some abilities and passive are shown to be either deadly or powerful as well as being a defining feature that we notice when we think of that champion (Fizz's Playful/Trickster, Jhin's 4th shot, Yasuo's double crit and dash). With Singed, you think of his poison trail and fling. Which, while those are what should be his defining characteristics, but they are backed up by his passive and ultimate so well (and even his W although it pains me to say that).
his passive is one of the worst passives in the game, but it helps him so much. Free stats are annoying and usually unnoticeable, which he 25% mana to health is definitely unnoticeable, but it gives him so much extra bulk. While you can balance him to give him that as just his base health, when you buy a righteous glory or for those who but rod of ages (I don't but some people might), that's free health. I feel that the passive definitely needs a change, but don't think that it's worthless.
Next is his ult, this ultimate is extremely powerful, but it has the same faults as his passive, it's free stats and not really noticeable (granted the cracking of glass/snow machine + the movement speed helps a tiny bit, but they don't show the stat buff). It can save his skin in so many circumstances, but it doesn't look the part of an ultimate at all.
I honestly don't have any real ideas on changes, if you give him a game-changing passive or ult they may have to change his whole kit, which I don't want.
His Q and E are his defining abilities, they don't need many changes, they would need to find a passive and an ultimate with equal power to what he has.
Sorry if this didn't do much, just me trying to explain why he's a bit tricky to balance. Granted I could be entirely wrong and not know a thing.
and now 
in the next patch.
without debilitating mana costs. There's some differences sure, but they share far too much to ignore
, 
, resistance item unless fed, 
w/e you feel like.
)
i'd like to see it scale with both mana and AP or mana and HP.