10 Doctor Who Villains You're Surprised Only Appeared Once
2. The Dream Lord
Arguably, the Dream Lord is the Valeyard - the embodiment of the Doctor's worst impulses. However, his genesis is different, the product of spores that find their way into the TARDIS systems. The Dark Doctor is an incredibly compelling concept, so much so that the Matt Smith era tried it again with much less success in Neil Gaiman's difficult second story 'Nightmare in Silver'.
It's odd that the Dream Lord hasn't returned in the series, there certainly appeared to be unfinished business at the end of 'Amy's Choice'. The availability of Toby Jones needn't be a problem either, as you could have a different Dream Lord for each Doctor actor. We've had opportunities for the Doctor actors to play darker versions of themselves over the years since 'Amy's Choice' but nothing that's captured the Doctor's self-loathing and anxiety.
It's strange that a story like 'Can You Hear Me?', which was all about mental health didn't reintroduce the Dream Lord as the embodiment of the Doctor's own anxieties and mental health concerns. Perhaps the introduction of the Fugitive Doctor put paid to that idea, so for now the Dream Lord is still out there, waiting...