I went through last night and found all the variants that I might miss if I play TI4 over TI3. I then went through and tried to judge the ease with which you could bring these elements over to TI4.
First, the list of things I (or others) might still like to have in TI4: (Note that this isn't an exhaustive list)
- Facilities
- Mechanized Units
- Leaders
- Representatives
- Space Mines
- Artifacts
- Mercenaries
- Distant Suns/Final Frontier
- Shock Troops
If you like them, Facililties, Artifacts, Mechs, and Space Mines all seem very straightforward to add to TI4.
Leaders might require slight modification, but they seem relatively straightforward as well. Personally, I felt like I was the only one who really used them in my group anyway. Also, the scientist might be slightly overpowered now that tech research has become cheaper overall. The discount would be a larger percentage of the original price now than it used to be.
Representatives wouldn't be hard to add, but I would worry about the balance implications. Once the new custodians token is gone, you have 2 votes per round. On top of that, the politics action seems better now than it used to, so I worry that if there was a lot more voting going on in TI4 than in TI3, the consequences of getting your reps assassinated could be much more severe. As such slight modifications could be in order, but that will take playtesting.
Shock Troops would be easy to add, but it's worth noting that the only consistent way of getting them (Hope's End) won't be in the game and even if it was, then it won't have it's old refresh ability.
Mercenaries. This one makes me sad because I love mercenaries, but just sticking them on the new Trade strategy would probably make it too good. That said, they would function just fine but their acquisition could use tweaks. I'll probably find a way to add them after playing a round or two of TI4. My group (and especially me) just like them too much.
Distant Suns/Final Frontier. All of these seem like they would work just fine in TI4. That said, I always felt the counters could be too extreme sometimes, so I will probably be removing some of the overly rewarding/punishing ones.
So why bother with all this and not just play TI3? Because TI4 seems to vastly improve the systems for trading, technology, and voting. I also expect there to be a general balance pass (fix Yssaril??) and the new components should be much nicer. Also, race specific promissory notes is a completely new feature ;).
I'm still personally a bit on the fence about getting TI4, but my list of variants I could bring over has helped somewhat in this decision. I just thought I'd share it with anyone else weighing the options! Feel free to add on to this.