Filling the roles that things like xatu and dugtrio provide for sun teams is definitely the approach we need to take. Another threat such as grass/dragon can be dealt with easily. Why? Because it will just be replacing another threat on the sun team. If we have CAP5 open up team slots though, we now have room for multiple threats which will be harder for rain to counter, no matter how good grass/dragon can be against rain. Now, the abilities and move pool of xatu and dugtrio are very important but we can play to their strengths still in this stage. What we need is something that can trap, control hazards, and be a pivot switch all at the same time. This is why I support dark/electric. This typing has the potential to be a great trapper with STAB pursuit, can keep momentum that as a bonus threatens rain with STAB volt turn, and shares few weaknesses with most sun teams. Another thing I like about it is that it has at least one super effective coverage against Rain's spinners in starmie and Tentacruel, and against spin blockers. I also like erisia's suggestion of ghost/electric as ghost is a great support typing and it provides a fighting immunity as opposed to a fighting weakness. However, this isn't a typing that is as inclined to trap effectively unless we were to give it the necessary ability to do so in the future. Also I think a dark/electric type would lend itself to sun more naturally as sun lacks a pursuit trapper while rain and sand could definitely utilize a defensive pivot/spin blocker. Also, trapping is already covered in rain with gothitelle/ scizor and in sand with tyranitar/scizor/magnezone.