I'm running a trickery domain cleric at lvl 3. I'm going to get atleast one more lvl before I consider multiclassing, but I was looking at a couple of options.

First mastermind, the languages, and learning local tongue could be great. Also there is the 30ft help action with all the other rogue things from 1st and 2nd.

Then there is warlock. The eldritch evocations of disguise self at will and maybe silent image at will could be awesome thematically.

I'm torn, both seem like an awesome theme to build on but I'm not sure which. Other ideas are also welcome too.

Eh, I played a trickery cleric with just one level of rogue. It didn't add a whole lot and the higher your cleric level gets the less important that dip will be. 

Based on my personal experience I would recommend to stay cleric. That is a plenty powerful class and there are really no options that would add a whole lot.

If you are insistent then maybe a few levels of whispers bard or illusionist wizard? Levels of a full spell casting class will hurt your spell casting less than a dip into rogue or warlock.