Does the time you spend inside the elemental wall affect the crit and damage? Or is it simply 1 roll per pass-through no matter how quick or slow you are per a given time period?
All of the points mentioned and discussed thus far are great and make for some fun number crunching, but this point stuck out for me. Speed/amount of time spent WITHIN the wall should matter, either through an additional roll for each round whilst stuck/passing through the wall, or a +## added to the roll which increases with each round whilst in the wall, or even an increase in Crit Severity. If one passes through a wall of fire, the target has to hold his breath and is getting burned. If he stays an additional round, things on his person may start to ignite, and he still has to hold his breath and keep his eyes closed. There should be one heck of a penalty for being stuck in the wall for a 3rd round given a round lasts roughly 6-10 seconds. Spending 18-30 seconds within a wall of fire can be pretty devastating.
Similar arguments can be made for nearly any elemental wall type.
Water: holding breath, being swept off feet/unbalanced by turbulence, disoriented and moving along the length of the wall rather than walking through
Air: Holding breath, or worse vacuum crits, dirt/debris swirling, eyes closed, unbalancing crits, smaller attacks from debris striking the target
Cold: Holding breath, cold, slower movement as armour freezes.
I don't know if RMU will handle elements the way RM2: Elemental Companion did by using Proto Elemental Material and PEM Poisoning. If so, any target caught in the wall and breathing in the PEM would also have to contend with the effects of PEM Poisoning.