To get things rolling I will begin with some key concepts of the core rules.
I see no reason to move away from the I go you go system.
As much as I feel that a d10 based system would provide much more depth to a tabletop game, for the sake of simplicity I think d6 should remain the dice type. Similarly I see no need to add or remove any characteristics from stat lines. The point of this project is to create a better more balanced game, not one that looks nothing like 40k.
I like the phases that most editions of 40k has had so I see no reason to change the type or order of phases significantly. I have mixed feeling about a separate psychic phase but overall it probably keeps things simpler if all psychic stuff happens in one phase.
As I said earlier, most random tables will be removed. This means that things like psychic powers and warlord traits will have costs (with the least useful/ most situational being free). However some random tables may remain like the random generation of wyrdboy powers.
Mission and deployment options hugely effect game balance. My idea for how to approach missions is that each player will have a deck of missions that they draw from. Each player will draw 3 cards from his deck and must chose one of those as his mission. he must only reveal his mission to the other player at the end of the game. Yes this might mean that one player is trying to remove an item from several objectives while the other player is only concerned about kill points but in the relatively small portion of the battlefield that a game of 40k takes place in the chance that both armies want to accomplish the exact same thing is small.
Special rules are an important part of the game, but in recent editions there has been a bloat of both USRs and unit specific special rules, one of my goals is to reduce the number of both of these.
Lastly (for now) saves, I feel pretty strongly that invul saves should not be re-rollable. I also feel that ignore cover should be accessible to all armies but that no army should have a super easy way of getting it.
I used an I go u go system myself, for my own game. But I think you might wish to consider other options more fully. You could look at LotR or Saga for examples. Force on force/tomorrow's war, too.
I think d6s are the way to go. Mathematically, using d10s instead of d6s does not create that much greater a spread in possible results. You can create much greater differentiation by following the initial roll with a second roll than by a imply increasing the size of the die type. 6 siders have numerous advantages - most players have buckets of them, and you can obtain them anywhere.
My problem with ignores cover, as a designer, is that it removes a major element of play. Essentially, maneuver becomes less relevant, as does interaction with the terrain. In my own game, I allow cover saves to be reduced, but never eliminated entirely. It keeps players interacting with the terrain.
Over all,I wonder if you are not thinking too small. The game you're describing seems like a minor modification of 40k. You should survey other games, sci if, or ww2 and modern, if you have to already. There is a much greater range of possibility to how a game can be constructed.
Edited by tdemayo, 29 May 2014 - 04:01 AM.