Hey Dan,
Riff writing (and writing in general) is such an interesting topic! Everyone will have their own approach but heres my two cents:
It can be easy to get distracted by what a riff logically should be - sometimes this can lead to analysis paralysis and you either don't write or you over judge what you've written.
The first piece of advice I can give for any type of writing is to treat it like 'play'. The idea is to get into a headspace where you're just trying things out, if they sound good - great 
Record it etc and if not, no worries. Kinda like how when we were all children, we would create fearlessly - we wouldn't be worrying about the quality of our crayon drawing etc or judging our abilities based on what we created. We did it because we could and because it was a fun outlet.
This first step allows you to get out of your own way and let ideas begin to naturally flow 
A writing exercise I always do with my students is to stick on a backing drum track (YouTube has loads of great Metal ones) and just let them practice the above step - having a consistent rhythm behind you can really inspire your writing, like jamming with a live drummer etc
The next step is quality control. Once a riff starts to surface, you can then - *gently* - begin to pass judgement on it. Its important to remember that it's only a riff you're judging and not your ability to write them. If you don't like it, you don't have to use it. But you've tried it out at least and you'll learn very quickly what you consider a 'bad' or 'weak' riff.
The more you engage in this process, the quicker the process will become and the stronger your riffs will sound. It's all a matter of fearlessness, open mindedness, & taste
Hope this helps!
