Well, I was thinking, when you learn Attribute Bonus skill, just place it's icon on location of Hero Abilities icon.
At first it won't be visible since it is going to be 'under' Hero Abilities icon, but when you reach level 25 and learn all skills, Hero Abilities icon will be removed and will get replaced with Attribute Bonus skill.
This way it would be same for all heroes when it comes to Attribute Bonus skill and that empty space is going to be filled when you learn all your skills.
Here I picked 2 heroes on random to give you an example of what it should look like.
Will show you how it should look on level 25 and compare it to current.
I hope you get what am I trying to suggest here.
If not, then say it and I will try to explain it as best as I can.