This mod for Crusader Kings II allows rulers to form larger realms with partners of the same rank.
With two adjacent allies of the same rank, you can form a new elective title of higher rank. Your co-founders give up their independence in exchange for the possibility of being elected as your successor.
League titles are titular. Since all vassals can vote in titular realms, you can invite additional members into your league after creating one.
In addition to allies, you can partner with potential allies (NAPs and close relatives) and effective allies (friends and tributaries). In all cases, opinion must be at least 26.
Install: from Steam workshop or elective_leagues.zip