Spell Stealer

Your ancestors were forced to undergo dangerous experiments in an attempt to make them immune to magic.  While these attempts failed, they have created a new type of Sorceror that has the ability to not just counter-spell a spell, but absorb it and learn to use it yourself.

Skilled Arcanist

When you take this class at first level you gain free proficiency in Arcana.

Extra Known Spell

When you take this class at 1st level, you also learn the 1st level spell "Absorb Elements".  This is  a special spell and can not be replaced when you gain levels. Instead, see the 3rd level feature "Steal Spell".   To cast Absorb Elements, you still not to spend spell slots,  but Absorb Elements is an 'extra' spell known, not one of the 2 you get as a first level Sorcerer.

Steal Spell

At 6th level, once per long rest you can use a reaction to steal a spell.  The cast must be within 30ft of them or the range of the spell (whichever is greater).   Doing this costs you sorcery points equal to the level of the spell you are trying to steal.

Make an opposed Arcana check, against the person casting the spell.  If the spell is not on your sorcerer list, your opponent gains +2 their Arcana check.  If the spell is too high for you to normally learn,  then they get an additional +4 to their Arcana check.   If you equal or exceed their Arcana roll, then your knowledge of "Absorb Elements" vanishes, and instead you now know the spell.   In addition, the original caster has as their spell counter-spelled.

Your knowledge of the spell lets you cast it using your normal spell slots.  This knowledge lasts at least until you take a long rest.  When  you take a long rest, you can choose to try and retain the spell.   Make an Arcana Check (DC = 10 +spell level, +2 if not on sorcerer list, +4 if too high for you to cast).  If you succeed, then you retain the spell until your next long rest (you can try to keep it again).   If you have retained a spell, you can not attempt to steal another spell that day, as you have used up the attempt.    However spell retention does not cost you sorcery points.

If you do not retain the spell (for any reason) you lose the spell known and regain the spell "Absorb Elements".

If you have stolen a spell that requires a spell slot you do not have, you can not cast the spell.  But you have counter-spelled it.  Also note a 7th or 8th level Spell Stealer can steal a 5th level spell on Sunday,  retain it, then use spell points to create a 5th level spell slot on Monday, allowing them to cast it.

Steal All The Spells

You can now use the ability to Steal a spell unlimited times per day.  It still costs you sorcery points equal to the spell you are stealing, and  you still must make the arcana check.

If you currently know "Absorb Elements", then replace it as usual.    If you do not currently know the spell "Absorb Elements", then you still can steal the spell, but the knowledge is fleeting.  How long you retain the knowledge depends on the type of spell you cast:

If the spell is on a spell list you know (either Sorcerer or any other list you gain from other classes), then the knowledge lasts for 10 rounds.

A spell not on any Spell list you know (including multi-classes), the knowledge lasts for 4 rounds.

If the spell is cast using a spell slot higher than you are entitled to, cut the duration in half.

In any case, you are still counterspelling the stolen spell.

Perfect Spell Stealer

At level 18, if you roll for initiative,  do not currently know "Absorb Elements" and have no sorcery points left, you gain +4 Sorcery Points.