I'm not sure which platform exactly, but if I was custom building my ideal combat side-arm, it would be...
-Automatic (semi, not revolver)
-Full/Service sized titanium alloy frame with a polymer coated grip, guard, and trigger:
(Low weight. Durable. Extremely rust-resistent. It won't freeze your hand off in low temps - and you can pistol-whip with it.)
-Double stack magazine in .40 to keep the grips slim
(45 would be too fat for me, but if possible to keep a slim profile, I would opt for .45. Although 9mm might be fine for SD, I would want a larger weight for combat situations.)
-Minimal/no safety (aside from maybe a drop safety of some kind)
-High visibility sights.
-ambidextrous mag release
-Easy take-down and build
-Proper grip angle (sorry glock)