Cool guitar riffs are typically a thing of pure imagination.
So in other words, the more willing to be mentally exploratory that you are the better your riffs might sound.
Not everyone regards themselves as being this creative, however I truly believe that creativity can be learned to a degree.
Take even the most creative player.
Could they create really cool guitar riffs when they first started playing? of course not, but why? because they did not possess the dexterity, musical understanding, and experience to do such a thing.
They had to learn what makes a riff sound cool.
This can be done by absorbing lots of music, taking the time to sit down and figure out things without jumping to tab always, and making an effort to focus on technique, and perfect the small things.
Less is more in my opinion, but everyone is a little different.
Never just bend a note or slide into it to make a cool guitar riff, always think about the smallest little accent that can make it better.
Its the guitar players who go the extra mile with their own playing that make a difference in their sound.
There are also plenty of guitar learning resources that are available where experts will show you how to learn and master guitar and create really cool guitar riffs based on their own experience.
This is usually a good way to get started learning guitar better and a heads up on what to look and listen for.