I actually prefer random charge distances as a game mechanic.
If you have no pre-measuring, then - sorry if this is a little cynical - the game is wide open to rubber-tape-measure-syndrome.
If you have pre-measuing, then fixed charge distances are easy to deny, especially with gunlines.
Random distances introduce risk management and baiting into the gameplay.
Whoops, I wasn't really clear about the full intent, here - full random charges would still be available as normal, but models would have the option to simply "take 6" (or less, depending on their movement value) for the charge instead. Fully randomized charges, with a minimum distance of only 2" and Overwatch to boot, has been such a punch in the gut for close combat armies, one more thing keeping them from engaging in their specialty while shooting units remain a "point-and-click" option that's rarely impeded by much.
Charges as D6 + Move or Move + 2d3 I could work with as being better
but the biggest change would have to be actual terrain rules, you need two broad types (which can be combined) - stuff that makes you harder to hurt like fortifications, brick walls etc - the save bonus I actually like, but to be honest would prefer something that dropped the strength of the attacking weapon - e.g. a wall drops S by one point, fortification by 2.
then you have stuff that makes you harder to hit like smoke.
area terrain that can provide its bonus simply if the shot passes over it (e.g. shooting over a wall between both units), without one unit having to be totally in it.
I would avoid terrain changing movement by more than dropping an inch from movement, maybe from advance and charging as well
this helps gunnies (who can dig in) and hurts them (by making them easier to approach)
The problem with adding to movement overall is that there's such wildly different movement values in 8th. You end up with either a lower overall charge range (tho a better "average" for many troops, I suppose), or ridiculously long charges for vehicles and flying troops. The "up to 6" solution takes out a bit of randomization for CC units - and ends up just a little short of the average random charge - without taking more laborious steps to limit this to certain unit types or movement values.
I definitely agree on terrain rules needing a huge overhaul, tho, and I like the idea of possibly lowering the S value of weaponry in different types of blocking terrain. It's something to look into, since the armor save modifier of so many weapons really makes "cover" a joke in this edition. The other option I'm looking at is making cover into a variable modifier depending on type, with "standard" cover being +1, but with things like walls and ruins and other solid fortifications being +2 or even +3 for some of the really tough ones. The obvious problem there is making nigh-invincible beakies, so the idea of modifying weapons strength may work better. Thanks much for the input!