We need more missions
The system from Hots could be adopted here without any issues. examples: win 3 pvp matches win 3 games while playing an assasin win 3 games while playing a support win 3 games on TT win 3 games of aram win x games of any rotating gamemode ...
Note how im avoiding stuff like "kill x creeps", "kill x epic monsters", "get x kills in a match", "accumulate x gold" etc. those types of quests incentivise people to goof around just to complete them.
As things stand right now we only have First win of the day and event related quests. Once you complete those you are left staring at an empty mission bar.