It's too bad they made the Sorcerer's mechanical niche metamagic. Being able to cost action spells as a bonus action, with a very limited set of spells, would be a really fun way to make Warlocks weird.
Maybe on a day off soon I will hash out the outline of an at-will Binder, as an alt Warlock.
Right now, the idea is ritual casting and cantrips but no pact magic or spellcasting feature, Eldritch Blast can be changed to a cone or line that gets bigger at the scaling levels but only goes off once, with a Pact of The Rod. The items associated with your Pact Boon are available as invocations, and thus you can gain more than one, and they are all spellcasting focuses and can gain enchantments via invocation (ie +1 to attack and DC when casting through your tome, rod, sword, or familiar).
You also gain 1 of 3 Ritual Tools at level 1 in place of pact magic. The Atheme, The Chalice, and The Bell. They each give at least 1 spell you can cast once a day, and can regain via some specific event or action, and a couple utility or minor benefit spells you can cast at-will, in addition to a passive benefit (Bell=external control/abjuration buff, challice gives you a spell slot or two pact magic style but per long rest, atheme= basically improved pact of the blade). You choose a second at level 5, and a third at level 11, and you can only have one active as your Implement at a time. Each might give more spells at certain levels, or not. Again, this is replacing Pact Magic.
Add in an Invocation that lets you use two spellcasting focuses at once, gaining a benefit when you do so. Maybe based on the type?
Your patron spells are each 1/LR, providing the bulk of your spellcasting unless you invest in a lot of spellcassting invocations.
I also want to fit in a Warlock's Curse of some kind, which might require pushing something else back further down the line, unless I can make it part of the Ritual Tools feature somehow. Perhaps each tool gives a different type of Curse you can use, and the Curse defines how you get back the spell that comes with that Tool?