A new tree for all heroes depending on their political ideology, this could add more choices so no hero would have the same set of skills, I guess I'm not the only one who tends to pick a hero of the same race I'm playing and heroes from different ideologies to occupy the senate.
For example the militarist ideology tree could have skills for ship production, troops.

The religious, a better cultural conversion, more influence, better hero healing time.

The Pacifist better luxury resource output, the industrialist better strategic resource output and industry, the ecologist better food production, etcetera.

Here you could also have special laws as skills and senator bonuses.