With power creep and gameplay changes made across the generations, usually coupled with neglection from GF overlords, old powerhouses have been made progressively weaker in newer metas. One particular jarring result is that a bunch of legendary pokemon lost their places in higher 1v1 tiers (or never occupied them, as in Regigigas's case) and fell down to RU, or even worse, to depths of NU.

In their rare display of wisdom, GF hires you to give upgrades to those forgotten legends, bringing them up to current competitive standards in UU or OU. While you're not allowed to change their typings or existing abilities, there's nothing stopping you from expanding their movepool and giving them new, more useful abilities (all of them have only one standard ability, so you can add a second one), or even buffing certain moves and abilities that are completely exclusive to this certain Pokemon (such as Regigias becoming perfectly viable if Slow Start is made less crippling than it is now).

Hard mode: try not to give them completely new made up OP abilities unless it's absolutely necessary, as with Articuno's case.