Well, if you're looking for a champion whose skillset doesn't feel completely repetitive, that implies you want someone who can change up their spell rotations based on the situation for different effects.
The main ones of those are:

Alternatively, you could look for someone whose skill set is fairly fixed, but who instead has differences in the way that they approach different situations, such as:

As for your other criteria:

As have been mentioned a few times, Syndra and Cass fit your other criteria quite well, but frankly their combos and approaches are fairly fixed.
Cass tries to hit you with her poison and then spams her E at you as much as she can. Her ult is a more interesting part of her kit, but the rest of it is fairly repetitive.
Syndra also mostly has one combo: hit you with some orbs, throw one at you to slow you, then stun you and use her ult to burst you down. You can change the steps around to throw the orb after stunning, but that's pretty much it.
Both of their kits are nuanced and take some practice to use effectively, but they don't really change up very much between uses.
You could also consider Lux, though again her spell rotation is fairly fixed (snare with Q, autoattack, hit with E, ult)
Another one you could consider is Orianna, she's somewhat similar to those others (though less bursty), and in her case the placement of her ball makes her abilities a bit more interesting.