5) Planet Upgrades
Though rarely used in the 3rd edition by anyone but me, I'd still like to see it in the 4th. What they did was increase the political influence or production value of a planet by one and you could only get one per planet. So it's a slow strategy of growth that does come in handy depending on what game style you're planning during the game.
4) Space Mines
Another rarely used item in the 3rd edition but still effective. Most time you'd plant them over Mecatol Rex to increase your chances of defending the planet. It also make the cruisers a little more desirable to build which I liked too.
3) Shock Troops
Another rarely used one but I think that's because you use a token to keep track of them instead of a figure. I'd love if they changed that and made this super commandos more relevant in the game.
2) Preliminary Missions
I know the vast majority of folks didn't care for this expansion but it sped up the game by giving players a way to make some quick points early in the game, thus speeding it up. (which helps solve TI's biggest complaint. Bringing these back would be easy and a great way to keep the time of game down for players. (though I could play all day)
1) Mercenaries
Man do we miss some Mercs! They were great to have in the 3rd edition and at least 2 of the players in every game would use them. You can't resist! The cost to keep them is high but their value exceeds the cost at times when they're used. I love it. This will now be the ONLY thing my gaming group will complain about not being in the 4th edition. Could I fit them in to TI4? I think so. Just need to put a reminder of the "5" tile to remind everyone the option is available. But I'd love to see it in the base game. I think it would sing.