I'm sure they will come. I just hope they don't make it so that they are only available through that lottery feature that they used in Set Sail for the Kingdom. (shudder). But that would still beat not being able to get Slifer and Obelisk at all, though.
That will never happen. I guess the God cards will be one time only cards, it would be kinda weird to own multiple copies of an Egyptian God card which would also contradict the anime.
Obelisk can appear as a one-of-a-kind reward from an event Kaiba Lv50 and Slifer from Yugi or maybe even Strings. However, although they're both more useful than Ra without support, Konami will probably launch then only when they become really outdated and useless...
Before they annouced the Ra event, I though that the Egyptian Gods would be unlockable non-event cards that you would get after completing a deadass hard mission and you would have to choose only one of the Gods. Or maybe a special unlockable Boss duelist that would give you only one God card depending on the LD that you used to beat him the first time (Obelisk if you use Kaiba, for example).