Personally, I like Failedlegend's idea around the SP problem and unlimited casts issue: have a larger echoes of power maximum SP limit which increases with your Warlock level, I'd even go one further: increase SP regeneration with level, or make it a level 18 feat.
I'm mixed when it comes to pets, but it would be a shame not to have a reskinned wolf into a hellhound, especially with all those summon and pets enhancements.
Shape-shifting ? The only shift I recall is into a Baelor (spelled right?) and even then I think it was epic levels I believe. But I'm ok if we don't have that, provided we have two very warlocky things:
1) unique spells. Come on druids and artificiers have unique spells so why not warlocks??
2) a way to have a more unlimited spells use than wizards and sorcs.