I recently made a lock because I was interested in the demonology tree, which seems refreshing and well designed. So I am coming from the perspective of not having much nostalgia.
But yeah a lot of spells and abilities show their age and look pretty awful, namely dark pact, unending resolve, and burning rush. Demonic circle is also a major eyesore to me. A lot could be done with them, especially spells like burning rush that are used so frequently.
I had the idea that for some spells a big bubbling Gul’dan style cauldron would plop down in front of the warlock as they cast. Or perhaps it could be an ability and used like a turret (you get a buff when casting near it, or you could interact with it and summon a random demon/buff from it like roll the bones…etc). Just random thoughts, but I think the cauldron idea would help emphasize class identity. Soul Well is kinda close but its a missed opportunity IMO to not use this idea more.