We've already got observation posts. Research the sensor branch, and then build a starbase with the sensor modules. Once you've put a few levels into them (the later tiers of starbase sensors are really powerful) the starbase will see anything in a huge radius. If you want to defend it, research some starbase fortification and you're set!
Mines are completely unfeasible in space. It's too large, too open, and mines would be very ineffective. Each square represents an (adjusted) parsec, after all... that is many, MANY light years across!
Stargates in the galciv universe aren't instant transportation, but more like non-portable hyperdrives. Traversing one still took months or years. What hyperdrive did was condense and alter the mechanics of it so that the same or better results could be gained on a ship mounted piece of hardware! So stargates would be a bit of a step backwards.

Okay, yeah, I see where you're going with that, and it would be crazy awesome... but that's a HUGE feature! Traversable wormholes or their artificial equivalents would require a huge overhaul of AI and pathfinding, so I doubt it'll make an appearance until something like GalCiv 3... if we're lucky.