A lot of people have complained about the fragility of walkers, and to an extent I agree. I personally would love to, for example, put out a squadron of Penitent Engines and not have them laughed at and destroyed with ease. And I know people think dreads are too defensively weak.
So, let's try and work out how to make them more survivable. Try and put your ideas out in this format, numbering them based on order of posting.
Idea #1 (from Melissia)
Summary
Make walkers harder to damage.
Specifics
Walker Mobility
-- Walkers may force an opponent to re-roll damage rolls. The second result must be accepted.
-- If an enemy rolls a 6 on their to-hit, they hit the walker's front armor instead of side armor.
---- Rear armor hits always count as rear armor hits.
Idea #2 (from Melissia)
Summary
Change Walkers to be Monstrous Creatures.
Specifics
This would require a rework of each profile and so I won't get into specifics, but in general hiere is what I would suggest:
FA10 = T5, 3 wounds, 3+ save
FA11 = T6, 3 wounds, 3+ save
FA12 = T6, 3 wounds, 2+ save
FA13 = T6, 4 wounds, 2+ save
FA14 = T7, 4 wounds, 2+ save
Along with Eternal Warrior and a special rule (inspired by the Sisters of BAttle special rule) that makes all Force Weapons (of all types) count as Power Weapons instead.