I think the 7 lights will be...
1. Sora
2. Riku
3. Mickey
4. Terra
5. Aqua
6. Ventus
7. Roxas
with the assistance from the usuals...
Kairi
Donald
Goofy
Axel
I curious why Terra hasn't resurfaced already considering that Master Xehanort is clearly back... And that What happened to him is similar to what happened to Terra...
also noticed that someone appears next to Roxas in the opening cinematic of 3D. who is it.
The line-up of characters we see in the Opening is only one possible combination of seven lights (not to mention that Donald and Goofy are there as well). Nomura specifically pointed that out in interviews.
While Aqua, Mickey, Ventus, Riku and Sora are sort of confirmed lights, Lea, Terra, Kairi and Roxas are all "candidates" provided as possibilities, but nothing is confirmed yet about the final composition of the seven lights.
It may even be that the actual members of the seven lights change over the course of KH III and the seven we start out with may not be the seven who we get in the end.
If we go with assistance characters who take part in the final battle without being a light I would say it's convenient to include Xion in that group as well.
Terra cannot "resurface" since Xehanort still has control over him, as he said to Mickey in DDD.
Master Xehanort came back but Terra is as of now still a "vessel", one of the 13 Darknesses on MX' side, most likely one of the still hooded members of the new Organisation we saw.
The current status of Terra's heart and why MX returned as old MX is left as an open mystery by Nomura on purpose, as he pointed out in the DDD Ultimania.
Like I said before, the line-up in the opening is only one proposal, it isn't the confirmed lights.
In the scene, Roxas stands between Riku and Terra, and the characters form a battle-line of (from left to right):
Terra - Roxas - Riku - Sora - Mickey - Goofy - Aqua - Donald - Ventus.