A defining element of the Rogue class, at least in terms of combat, is the Sneak Attack. In 5E, this sneak attack can be used once per turn against an eligible target. Sneak attack damage can be applied to any 1 attack that lands that turn, whether that attack comes from an action (typically main hand attack) or a bonus action (typically off hand attack).
In BG3, Rogues can only sneak attack on an action. This gives them much less opportunity to land a sneak attack and thus limits their combat effectiveness. GWM fighters, frenzied barbarians (which have no exhaustion penalty in BG3), Smiting Paladins all substantially out damage a rogue. Giving rogues the ability to apply sneak to offhand attacks (again still 1 sneak attack /turn), would help bridge the gap a little bit and make them more viable in BG3.
In terms of how this could be implemented, a toggle to 'attempt sneak attack' is one option. When toggled on, the next attack that lands that meets the criteria has sneak attack damage applied. Any further attacks that turn would not be eligible. If it is easier, adding bonus action based sneak attack 'class action' could work too. The existing sneak attack action and this new 'bonus action sneak attack' action would be disabled for the turn if the other landed already that turn.
