I didn't grow up playing the Pokemon TCG (especially in the era where Light Pokemon were a thing {that being the WoTC Era}), but I remember Light Pokemon being meant to be the defensive counterparts to Dark Pokemon. They were apparently pretty half-baked so only 2-3 of them were ever good, but I just thought of one small detail that could be added if they ever come back.
Light Pokemon have altered Weakness/Resistance mechanics. While most Pokemon take 2x the damage when hit by a supereffective attack, Light Pokemon only take +20 instead.
This is a very small change, but in the modern era of the TCG, it could help these Pokemon be better at Defending and Supporting their allies, while not doing anything to make them overly-powerful. If a Pokemon has 120 HP and takes a supereffective hit with 60 Power, they take 120 Damage (a One-Hit Knock-Out). Alternatively, if a Light Pokemon in this scenario were to be hit by the same attack, they would instead take 80 Damage. This means that they survive the hit with 40 HP to spare.
Now this isn't very reliable (Playing towards Super Effective Hits stopped being a thing around Gen 5), but it's a better alternative to requiring them to run multiple color energies on their attacks and giving them no support cards to do so.