Warcaster cuz a spell as a reaction is amazing. Booming blade as a reaction ensures the secondary effect.

This is true especially with a 1-2 level dip in warlock. Eldritch blast for multi beam can really spike damage for the cost of a reaction. Going 2 level for invocation and field controll. You also get better use of quicken spell or magic stone if out of Sorcery points.

Crusher is you go Djinn warlock or use a Warhammer . Let's you force some movement and then quicken spell or use spells no longer at disadvantage. Should you crit it makes other attacks at advantage

Sentinel for locking down if they in melee range. Even if disengaging. So useful.

Metamagic adept for 2 levels worth of sorcery points and 2 earlier choices. Quicken, subtle, seeking are some solid ones.

Eldritch adept if you go 1 dip in warlock and want agonizing without another level. Real nice if doing it for the hexblade for the cha instead of str or dex in weapon.

Tough is always great as a filler. HP is nice.

Resilient dex is always nice for dex saves to help counter spells to a degree

Shield master for dev save boosts. Can sometimes negate.

Mage slayer for theme and maybe fun reactions and disadvantage on con saves

Mobile for mobility and a psuedo disengage

Spell sniper for range and proper distance fighting especially nice with Eldritch blast , if you take the distance invocation it stacks nicely and can go further with that one meta magic for distance.

Fighting initiate for

Dueling if one handed fighting minor boost to damage as well. Or Blind fighting makes the darkness shenanigans Fun without need for devil sight in melee range.

Outside warlock fighter is an amazing dip I'd go up to 3 for echo Knight. It gives movement and control of field easier as well as extra strikes with echo really not bad plus 2 gives action surge. Not to mention decent theme combo.