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:
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...