It sounds like you not only do effects, but also tools (given the road tool reference).
Idea 1:
So perhaps making a tool for various cloud setups would be nice? Both clouds that you don’t interact with, like those of a skybox, and clouds you DO interact with (fly through in an airplane and such). Big clouds, small clouds, stormy clouds, clouds with lightning streaking through them and maybe out of them, clouds that change over time maybe controlled in blueprint, etc. For clouds that you can interact with, having some sort of short lived trailing cloud/smoke effect attached to any character that moves through it would be cool (like they are dragging some of the cloud out with them as they exit).
Idea 2:
A lightning system that connects to objects near its point of origin, as seen here: Lightning effect using scene-probes - YouTube
Idea 3:
A flamethrower tutorial that does some of the more subtle effects of actual flame, like how fire sort of ‘curls’ back up and on itself when it hits the ground at force, as seen here: Flamethrower FX - YouTube
Idea 4:
A tutorial on how to use the material editor to get the most out of our fx work, like maybe how certain math functions can be used to produce certain designs or patterns, how certain expressions like depth fade can be used in unusual ways to get cool results, etc.
Idea 5:
Maybe a tutorial on how best to connect variables directly into a particle system from outside blueprints and such, and maybe a bit on particle instancing?
Idk, just a few questions i’ve had floating in my head for a bit. 
Looking forward to your tutorials, whatever they are directed at.