BEWARE MASSIVE SPOILERS FOR MGS4 ARE IN THIS POST!!!!
These are my own opinion and are not official in any way.
Who will develop it?
Well we all know that MGS5 is on its way, Kojima has gona on record he wants to act as a producer on the game and give the role of director to someone else. Yeah right, that is very unlikely, firstly MG2, MGS2, MGS3, MGS4, all those games he has gone on record saying that he will not be directing them, but after 'lots of fans asking him to' he always leads the development once again. Besides I am sure Konami will pressure him to lead development on 5 as his name will sell more copies of the game.
When will it be announced?
I am guessing TGS this year, maybe a teaser trailer or something similar to that. Failing that E3 or TGs next year. E3 this year is too soon, they are unlikely to announce anything Metal Gear except for Digital novel 2 coming to the US and Europe. History shows that MGS games seem to be announced very early in their development cycles and therefore very shortly after the last MGS game was released.
Will it be called Metal gear ***** or Metal gear Solid?
Most likely it will be Metal Gear Solid 5, Kojima and Ryan have both called the follow up Metal Gear Solid 5 in past interviews. This also may point to the fact it will be related to previous solid games. That is very likely because i think they know that many people will be disapointed if MGS5 was completely different to previous MGS games apart from the name.
When will it be set?
Most likely after MGS4, Konami learnt a lesson with MGS3 in that they will sell bucketloads more if the game continues the story set in previous games. Since MGS4 ties up alot of loose ends of the Snake saga it is unlikely that Kojima will choose to go back into the history of MGS. Setting the events after MGS2 also does not make much sense as there is not alot of material to work with and most of the plot would have been given away by the revelations in MGS4.
What will it focus on?
I think the focus will not be on Snake any longer, he will definitely appear in the game in some form or other. The story will definitely tie into what happened in previous MGS games. I reckon the game will be set when the world is still recovering from the downfall of the patriot AI systems at the end of MGS4 and the collapse of the war economy. The world is probably in turmoil because of the destruction of the AIs which would be a perfect setting for a new MGS game.
Who will we play and who will return?
Either Raiden or a new charachter of foxhound heritage. Raiden makes the most sense as his story is still not really finished and he is pretty badass now as well. Snake will be in the game but he will not feature as heavily as in MGS2, maybe a few cameo appearances. Otacon, colonel and Meiling will probably be in the game as well.
Other charachters will probably return like Sunny, Rose etc. The rat patrol will probably be there to provide comic relief as well. The game will definitely introduce some new charachters such as Dr. Magnar (who is talked about but not shown in MGS4).
I have a feeling Big Boss may make a return as well, he has 'died' so many times its a bit stupid so who knows he may come back. At the end of MGS4 he says he knew about the new foxdie but he still comes to see snake at the graves, I doubt he would just kill himself off like that after all the effort and sacrifices him and the other Patriots loyal to him made to bring him back to life. Suicide pill anyone?
Also I reckon Naomi Hunter will make a return as well, Otacon has got to have something good happen to him once in a while right? besides in Metal gear terms unless you are properly squished, blown apart or shot to peices you are most likely not dead. Also she is a bit of a sneaky bitch, who knows what she had in plan when she 'died' in MGS4, her charachter's motives were never properly explained (infact neither was Vamps, wouldn't he want to kill liquid/ocelot after Liquid Ocelot killed Fortune in 2?).
Also there will probably be another Metal Gear (we didn't get a new one in MGS4). Perferebly a HUGE one that is pretty fucking awesome, unlike Ray which was kind of stupid looking compared to the awesomeness that was Metal Gear Rex