I like the concepts, but the details would definitely need tweaking. Giving the Legionnaire an effectively free and instant taunt on top of his already amazing repertoire would be a little excessive. It would be better than Counterstrike in all its glory. You'd just run around the map with Walking Fortress, periodically toggling Shield Wall to draw aggro. The incoming attacks would let you have nearly constant usage of Walking Fortress...even more so than current builds.
1/10th would probably be way too little, considering blocking already negates the incoming damage; the Guard generated is just an added (flimsy) bonus. Being able to plug zone 5 doorways at very little cost comes to mind. 1/2 the current stamina cost or even 2/3 would be a more reasonable change. Although I think the issues with Guard lie much more in its damage absorption capabilities than its generation.
Sure so some adjustment would be required to prevent people from spam toggling for the taunt.
Consider:
1) When toggled perform an AoE taunt but add a timer that prevents further toggle taunts for 15 seconds.
2) The taunt is a directional cone AoE, as to only taunt enemies directly in front of the tank.
Regarding the stamina consumption, the issue is perilous difficulty enemies and scaling effectiveness of the skill. Perhaps the answer is to have the skill recognize the difficulty and scale accordingly. That is a shield wall blocks as effectively on routine as it does on perilous, otherwise the skill is useless for the standard reasons.
Remember shield wall is a very special skill, it has nearly zero offensive capability (other than reflecting), stops stamina regen and greatly slows you (even with chevalier step). Since we only get 4 skill slots this skill needs to perform better on perilous, further "Balanced" combat is a dead skill if you dont take shield wall as you need to block in order to trigger the opportunity for stamina regen.
TL;DR - Shield Wall should be a hallmark lego skill, but it under performs due to stamina costs associated with damage mitigation and poor guard generation (per hit). This skill becomes completely obsolete on some threatening, and all perilous missions.