I would rather "spend" spell slots to "augment" any at-will powers.
For example, say Telekinetic deals 1d8 force damage at will, spending a slot-4 might boost it to 5d8, maybe with additional range and area as well.
Such a design is really a cantrip that additionally allows spell slots to boost it.
Consider a "Psychokinesis" cantrip. Something like the following.
PSYCHOKINESIS
Telekinesis cantrip
Casting time: 1 action
Range: 30 feet
Components: none
Duration: 1 round, allowing other actions the following rounds to continue the effect.
You can make fly a Tiny willing creature or an amount of mass within a Tiny space to anywhere within 30 feet of you. You can also manipulate the target, such as moving limbs in various directions, or causing a malleable substance, particles, or a fluid to form complex still or moving shapes within the Tiny space.
Augmentation:
You can use a spell slot to increase the size of the psychokinesis effect. The slot level augments as follows:
S0 (cantrip) Tiny
→ S1 Small
→ S3 Medium
→ S5 Large
→ S7 Huge
→ S9 Gargantuan
Notice, that Wizard might be able to choose this psionic cantrip too.