Singed is a Scientist and a Chemist

http://lolalytics.com/image/generated/share/Singed.jpg

Why not make his ult a bit like But instead of upgrading his abilities he can choose to drink from 3 to 4 different kinds of potions To change the course of battle with just a sip of a special brewed potion!!!

http://www.indian-heritage.org/kidscorner/comics/astatw.jpg

Or option 2 Give singed 3 or 4 kinds of potions where each potion supplements one of his abilities The 4th potion could be what he has now or something else entirely!

http://orig03.deviantart.net/d4b6/f/2016/136/2/1/211f8ebb66395ade7e5355ba752ad996-da2rs90.jpg

Edit:

https://hydra-media.cursecdn.com/hearthstone.gamepedia.com/thumb/7/7e/Kazakus%2849622%29_Gold.png/200px-Kazakus%2849622%29_Gold.png?version=75c35b5da20508f8e4f94102e3c41f87

Option 3 could be done Where singed brews a potion combining all the effects he desires
But that seems a bit hard to balance so...