So Singed....
I honestly think Singed really needs a passive rework... not an overall one. BUt he's forced into these ludicrous builds to make use of his passive and then has zero move speed or tankiness to support the minimal bonus HP from it.
I noticed this in Ascension(Yes I know, not a good benchmark) But i had a ton of itemization that would be considered "good" for singed to have but it means literally nothing when champions with merc treads and spell pen boots simply outwalk singled with swiftys and ult running, and then when they turn on you after you putter out, you have zero tank items to survive with since the new items provide none when trying to even go RoA into semi tanky items, which you then give up having Rylais, or the Hextech slowing item.
I like playing SInged, I think he's a blast, but its next to impossible to play at the moment.
Edit: As for making a suggestion to better Singed, outside of adjusting his abilities to not be so mana based for a build that has little mana OR if someone chooses to stack mana. Mana should turn into Armor and MR or into Move Speed, something more viable for him as a champion rather than an awkward 300-400 bonus hp that does nothing.