I don’t think this has been suggested, apologies if it has and I’m being redundant, but to take a page out of the guitar world (another mono instrument), a wet-dry-wet setup sounds glorious. This is more of a “process” than an “effect”, and perhaps applies more to recording (it is certainly easier to do in recording).
Basically run your synth mono dry and in a parallel effects chain add stereo effects. This has the advantage maintaining of a pure, unaltered, mono signal path. When you immediately hit a stereo effect (chorus, flanger, phaser, etc.), the fundamentals of your original signal aren’t as strong. The wet-dry-wet routing largely solves this issue.
In the guitar paradigm, you’d use 3 different amps. One for your dry tone, one wet/effects left, and one wet/effects right.
So a dry mono signal paired with any number (or combinations) of stereo effects:
— chorus / flanger
— stereo phaser
— micropitch
— stereo notch filters
— slap-back delays
The stereo effects add width and space to a core tone that remains fundamentally rock solid.