So far this is what is on record.

EDIT:
1) From what I have just confirmed from various reliable sources in japan, there are no official records for the next series. EVERYTHING so far mentioned is ONLY specualation. The whole Fukuda interview (I posted it on the upcoming/spoiler thread) was just him going on the record for how difficult the series was to make (explains his LACK of creativity). I haven't had the time to do a full translation but it is basically him explaining his experience with Destiny and nothing more!

I will not post any official material until it is a reliable/confirmed source

I would love to see a third series, however, there have to be drastic changes. Not only that, sequels always have to outdo the predecessors. So the third installment better be much BETTER than SEED and have way less recaps and used footage than DESTINY. GSD did not surpass SEED in any way for me, other than a much more upbeat/fast paced first half. The second half (and most important for me) was very dull and depressing.

If a third series comes out, I would rather it be several years (like 20) later where Kira/Lacus/Athuran/Cagalli/Shinn would play a minor role if any at all. The series needs new faces without interference from old casts. Otherwise, it will be doomed as was Destiny. The problem with Destiny was bringing back the old cast of SEED. They should have never done that. However, if this trilogy is going to live up to the SEED originality then Kira and Company coming back is a no brainer. It begins with Kira and ends with him in essence.

If the third series is with a whole new cast without any interference by the old folks, I believe it would have the potential to outdo SEED. But if we're going to have the trio back (Shinn, Athuran, Kira) and on the same side, then it is MAJOR overkill. Along with a new cast and a latter setting, a whole new storyline. GSD was based on SEED in many ways, a lot of things were TOO similar. The whole group capturing the other sides mobile suits, etc.

I'll have more to add on my thoughts, and I will post more official news when it comes my way.

EDIT: When Fukuda does the interview, I will post it with a translation if need be