i used to run a pro 3, here are my comments
save a bit of weight if you can, but mainly go for stiffness. there's no point making a chassis 60g lighter then having to add 60g weight to make it legal (except weight distribution, but you won't need 60g lead to fix weight distribution problems). so yeah go mainly for stiffness, save a bit of weight if you can, move the electrics forward or make the positioning of them adjustable, maybe some other design changes like that.
and yes, we all need a topdeck, can't live without one, but make sure we can get out packs in & out easily, like PRP did.