The Problem with Singed Passive
As you may have notices, in high level play (the very few times he is used), mana isn't built on him in spite of his passive being :
Empowered Bulwark: Singed gains 2.5 HP per 10 Mana. (25% of total mana as hp)
So this is why:
Early game: If managed correctly you can get by your early game by not building mana, he has a core item that he must rush (
or defensive items) which means he can't get the mana items there are for him to get 
or 
, both of which require to be rushed.
Mid-Late game: With his base scaling mana, he already gets more mana than he will spend by the mid and specially late game, so here lays another issue: by the time he can build mana items, he no longer requires the extra mana.
So long story short, the general problem is: his passive is made around building mana but he doesn't need mana, using mana items to get hp is more inefficient than building hp directly and in the only moment of the game he could use the mana he is basically required to rush other items.
My suggestions if Riot want's 
players to buy mana (or have it as a viable option) would be making it more tempting, making it more an alternative (and VIABLE) way of getting other stats, unless they want a passive that doesn't have an impact of his play/buildstyle:
**1- **
Empowered Bulwark: Singed gains 5 HP per 10 **BONUS **Mana. (50% of BONUS mana as hp)
Ok, this would make it so you don't get the slight hp you get from your base mana (which is basically just glorified hp scaling), but get way more hp from building mana items, making building mana something that makes an impact, i mean, as something with Singed passive, shouldn't you want to build 
over items like 
?, but we see that is not usually the case.
His highest winrate builds don't even have mana, and just 
tends to be enough to survive earlygame, and that is even sold afterwards so not really used for his passive.
2- And i like this a tad more: Empowered Bulwark:
Singed gains 1 HP per 10 totalMana. (10% of total mana as hp) Also, Singed gains Tenacity scaling with his bonus mana (probably in a logarithmic way like armor works).
Because as , pretty much if you are not moving you are not doing anything, gaining tenacity from mana would certainly make it tempting to get build it.
