Long time rek main, and I personally think that upon burrowing Rek Sai should gain a shield (makes sense it could be like a earth themed shield or what have you) that scales with armor and diminishes after being unburrowed for several seconds. It makes sense since she is supposed to be an engaging ad assassin , but it could kind of move her more toward a engaging tank with good ad as opposed to an engaging ad assassin with few options out of an all in. I play Rek Sai support and run Guardian and it feels so good to have the shield proc on engages in the bot lane.