I would love a rule that you can only summon from the same Allegiance. For Example, a Chaos Sorcerer with the Mark of Nurgle can only summon Daemon's with the Nurgle keyword. An unmarked Chaos Sorcerer could summon what it likes. It would fit the fluff better.
But then giving Khorne Priests an ability to summon Daemon's with the Mark of Khorne would balance out the not being able to Summon Daemons with the Khorne keyword. Base it off number of models killed in the previous round, a Blood Tally. Every 2/3 models dead = 1 summon point. Make the summon a 2+ on a D6, 1 fails and priest takes a mortal wound, andyou could summon a Bloodthirster if 20/30 models died the previous turn.... Now having lots of Bloodreavers to throw at the enemy lines to increase the Blood Tally would be a good gaming strategy. Fluffy and fun!
Sadly this last point is a Warscroll change and not a GHB change (Perhaps in the Disciples of Khorne Battletome!?)