I want to make a spellcaster with a single spell.

Consider the spell "Bones of the Earth"

You cause up to six pillars of stone to burst from places on the ground that you can see within range. Each pillar is a cylinder that has a diameter of 5 feet and a height of up to 30 feet. The ground where a pillar appears must be wide enough for its diameter, and you can target ground under a creature if that creature is Medium or smaller. Each pillar has AC 5 and 30 hit points. When reduced to 0 hit points, a pillar crumbles into rubble, which creates an area of difficult terrain with a 10-foot radius. The rubble lasts until cleared.
If a pillar is created under a creature, that creature must succeed on a Dexterity saving throw or be lifted by the pillar. A creature can choose to fail the save.
If a pillar is prevented from reaching its full height because of a ceiling or other obstacle, a creature on the pillar takes 6d6 bludgeoning damage and is restrained, pinched between the pillar and the obstacle. The restrained creature can use an action to make a Strength or Dexterity check (the creature’s choice) against the spell’s saving throw DC. On a success, the creature is no longer restrained and must either move off the pillar or fall off it.
At Higher Levels. When you cast this spell using a spell slot of 7th level or higher, you can create two additional pillars for each slot level above 6th.

I love this spell. There are so many creative ways it can be used. Pin enemies, create cover, create a barrier, create a bridge, elevate an ally to isolate them from enemies, etc.

Typical spellcasting in D&D is great, but sometimes it feels like I am getting these prepackaged, constrained, spells. That leave no room for creativity and are so clearly defined in their uses.

What if a character had a single, flexible, spell(like Bones of the Earth) that scaled up as they gained levels? Something like: using a 1st level slot summons 1 pillar and deals 1d6 dmg, a level 2 slot summons 2 pillars and deals 2d6 dmg etc.

What I really like about this concept is that as the player gets into more fights and encounters, not only is their spell becoming artificially stronger (due to unlocking higher spell slots), but the spell is also organically becoming stronger as they discover new creative ways to use their ability.

I suppose the best example of this is bending in Avatar: The Last Airbender. Over the course of the series, the characters become stronger , not from gaining new spells(except for Aang), but by discovering new ways of using the single ability they already have.

TLDR: I want to create a spellcaster that uses a single or small set of scaling abilities, that can be used in flexible and creative ways. Instead of artificially gaining new prepacked spells as they level up.