I really do not understand this line of thinking.
We have a game that is not dominated by any single class -- regardless of what people might claim. And we have a game that has a declining population. Unnecessary changes to the game create unnecessary problems.
There is no "monk splash" problem. The wisdom bonus to AC is part of the AD&D game. It is no more of a problem than "paladin splash" or "rogue splash" or any other "splash". Each offers something that is beneficial to the character design. But, in taking any splash you also give up something in exchange. This is not a design problem and it is not an implimentation problem. It is a "problem" only because some people are not personally happy about the success of certain builds. That is not something that deserves to be fixed by Turbine.
There is a Tempest problem because the AC benefit stacks when it should not stack. The most reasonable solution is to make it a shield bonus. That causes it to not stack with shields. But, before we go fixing the problem we should ask ourselves, "How big of a problem is it really?"
I simply do not see TWF Tempest builds going with shields -- the loss of attacks and DPS doesn't justify the marginally higher AC. So, while it is an implimentation problem it may not be big enough in actual practice to justify fixing.
If Turbine feels it needs to be fixed the answer is to make it a stacking bonus to TWF defense. Then it only applies when TWF and not when equipped with shield or two-handed weapon. It should stack with the shield spell as that is a magical effect that replicates the armor protection of a shield -- it doesn't actually create a shield that needs to be equipped and used.
Other solutions are not appropriate.