A bold proposition of a new army rule. Ridicule me if you want.
Army rule: Inquisitorial Enlistment.
If your Army Faction is AGENTS OF THE IMPERIUM, you can include any IMPERIUM units (excluding TITANIC, EPIC HERO and DEDICATED TRANSPORT units) in your army even if they do not have the Faction keyword you selected in the Select Army Faction step.
In this case, the maximum number of IMPERIUM units you can include in your army depends on the battle size, as shown below.
Character. Battleline. Other.
Incursion. 1. 1. 1
Strike force. 2. 2. 2
Onslaught. 3. 3. 2
Units added this way may make up no more than half of the total points limit of your army. (500 in incusion, 1000 in Strike force, 1500 in Onslaught)
Units acquired this way gain the ENLISTED keyword.
ADEPTUS ASTARTES ENLISTED units gain the ORDO XENOS keyword.
ADEPTA SORORITAS ENLISTED units gain the ORDO HERETICUS keyword.
GREY KNIGHTS ENLISTED units gain the ORDO MALLEUS keyword.