How does this affect Wall of Fire mechanically in terms of line of sight if it took on an element other than fire. And how is this supposed to make sense thematically.
The Transmuted Spell Metamagic does nothing but change the damage type
That is the only thing this Metamagic does and all the other effects of the spell do not change. So the wall of fire is still a wall of fire and is still opaque, it just, through the power of magic, now deals a different type of damage.
When Transmuted Spell is used the only thing that changes is the damage type, if something else were to change, the feature would say so.