The Binder actually has all the ideas it probably needs to really shine.
It's a summoner currently, but I think that the series of classes I'm building will be more focused than official 5e classes, so it might end up somewhere between the Witcher, an occult magician, and the Abhorsen (Garth Nix series of that name).
tier 1: pact magic with prepared spells, spellbook, ritual casting, and cantrips. You automatically know and always have prepared Find Familiar and Unseen Servant.
Ritual Tools (blade, bell, bowl), you choose one at low level, but gain a second around 11 and the third around 16. Each one has bonus spells known, and a fairly passive benefit.
Vestiges of Power. Somewhere between Invocations and Infusions.
Subclass (Seal maybe?, like Seal of XYZ), determines what you do with the power you summon and bind. Classic summoner, bind power into your body to hulk out, bind power into a weapon, etc.
Tiers 2+ Spell slots up to 5th like warlock. Must choose Vestiges to get higher level casting 1/day and unlock ability to learn that level of spells (ie, Vestige of The 6th Circle, grants 1 6th level spell slot, learn 1 6th level spell, and gain the ability to add 6th level spells you find to your book).
Most of the design space is in the tools, vestiges, and subclasses.
I'm not sure what the best other classes to develop alongside these are.
Swashbuckler, Captain, Alchemist, a few others...