This is coming from the perspective of a play through as a Great Old One Warlock:
1. It seems to me that a warlock's patron with whom there is a Pact would not be very happy about another entity trying to claim the PC Warlock's soul (essentially), and I'd imagine there would be some sort of tug-o-war between Patron and tadpole, which I can only assume would be unpleasant for the Warlock to be the rope in this scenario.
2. Give Warlocks encounters like dreams or visions when reaching milestone character levels, like level 3 for example - or when they choose a boon.
3. A warlock's patron must have some plan or goal it needs to enact/fulfill, right? So why not put it in as a quest line for the warlock play through? One type of quest line for The Great Old One and another for the Fiend. (I know - Probably a crap ton of work!! But hey, I'm asking)
4. Basically, there's nothing thematically important/interesting about being a warlock.
