There are many ways to distinguish sub-classes, including the existing feats but the key is to build to a concept rather than to build for optimisation.
I played in a group that had two warlock-rogues (fey/arcane trickster; fiendish (re-skinned as a shadow mage by mixing subclass features)/arcane trickster; and a shadow monk with eldritch blast from Arcane Initiate feat). Because Eldritch blast is so useful it did feel like all three of us spent a lot of time just blasting stuff every round and it made us feel very samey in combat, despite different spells, flavour, and abilities. Admittedly, the monk had a lot of awesome tricks instead of a wide array of spells and my tome warlock rules outside of combat, with at-will levitation, mage hand.
In the end, I re-tooled my warlock, went Swashbuckler rogue in place of Arcane Trickster and picked up some wizard spells from Arcane Initiate as compensation. I ditched Eldritch Blast completely. I know it's amazing but I just had to make my PC more distinct so my go-to ranged spell is now Firebolt.
The point is that player choices can help to make the sub-classes more distinct but there are rarely multiple optimal choices. Build the concept IMO. Obviously, more feats generally will help with this! I hope the new book has a few.
I played in a group that had two warlock-rogues (fey/arcane trickster; fiendish (re-skinned as a shadow mage by mixing subclass features)/arcane trickster; and a shadow monk with eldritch blast from Arcane Initiate feat). Because Eldritch blast is so useful it did feel like all three of us spent a lot of time just blasting stuff every round and it made us feel very samey in combat, despite different spells, flavour, and abilities. Admittedly, the monk had a lot of awesome tricks instead of a wide array of spells and my tome warlock rules outside of combat, with at-will levitation, mage hand.
In the end, I re-tooled my warlock, went Swashbuckler rogue in place of Arcane Trickster and picked up some wizard spells from Arcane Initiate as compensation. I ditched Eldritch Blast completely. I know it's amazing but I just had to make my PC more distinct so my go-to ranged spell is now Firebolt.
The point is that player choices can help to make the sub-classes more distinct but there are rarely multiple optimal choices. Build the concept IMO. Obviously, more feats generally will help with this! I hope the new book has a few.