Apart from Tiga, it is not yet confirmed who the other Ultras would be, but here are a few of my choices, in no particular order:
As someone said, I would love to see Daigo meet Hayata onscreen. (Actor Susumu Kurobe is the father of the actress who played Rena, Daigo's love interest (or wife, depending on what timeline we're talking about).)
For obvious reasons, I want Mebius in this movie, although that might be unlikely... (George's actor will be busy for sure, don't know about the other actors...though, does anyone know Shunji Igarashi's schedule after Shigeshoshi?)
The article mentions Yusuke Kirishima (Himeya - my other favorite reporter). If he is back for the movie, this will be even more awesome.
4. Seven (Original)
I don't know how likely Seven will appear in this movie either, since they have Seven X as a series already. But I hope he'll be back anyway.
(I'm not naming the guys whose full series I haven't seen, even though I might like them too.)
ETA: I got this list of possible movie actors from reading the thread at UltramanLah! (Don't want to paste the Japanese text because I'm not sure if it is accurate.)
Ultraman Tiga (Hiroshi Nagano)
Ultraman Dyna (Takeshi Tsuruno)
Ultraman Gaia (Takeshi Yoshioka)
UltraSeven (Kouji Moritsugu)
Ulraman Ace (Keiji Takamine)
GUYS Captain Ryuu (Masaki Nishina)
ETA 2 (and probably my final edit): Official site doesn't seem to have anything at the moment other than a pic; I don't do too bad with the guessing, although my hope for Nexus seems to have already fallen off the window. (And I just noticed that it used to be the site for the 2004 Ultraman movie and subsequently the 2006 Mebius movie; way to go Tsuburaya for hanging on to only one domain. If this pattern continues, will we get another movie in 2010?) I'm probably not going to be posting any further development, mostly because I can't keep up but also because this post is starting to fall off f-lists. At any rate, information seems to be posted all over the place by now.