I'd like to see E-Liter 3K/Splash Wall/Kraken.
E-Liter 3K because it's got the longest range in the game and good power too, but has a long charge time, which can be alleviated with the Splash Wall and Kraken.
Splash Wall because you can hide behind it and snipe through it (not having to worry about being flanked while sniping is something I wish I had, so...), and you can deny entrance to areas with it too (the center opening to the middle of Urchin Underpass is begging for this, but I can't seem to position it right; I'm sure there are other areas, but that's the one I think of)
Kraken because you can use it to run away (invincible and speedy), infiltrate areas (since it creates ink as you move, you can get around easily while you're a super-squid) and splatter people (not really a priority, but if you get the chance to drill through people, go for it).
The idea here is that you have all the range and power of an E-Liter 3K, Splash Wall for defense, and Kraken for mobility and more immediate defense.
Really, I'm not sure if this is perfect (maybe replace E-Liter with a Squiffer for better firing speed, if the power/range loss isn't too bad), but I'd like the chance to find out someday (hopefully on the E-Liter 3K Custom the strategy guide says will come soon).