1. 4D. As well as using a holographic projector of some unknown futuristic design, it will also require the player to own and operate a mini-TARDIS for the duration of the game. Wearing a stupid scarf and running around shouting "IT'S THE WEEB OF TIIIIME!" will be optional.
2. No, it will be operated by the aforementioned mini-TARDIS, with an umbrella sporting a question-mark shaped handle being used as a secondary control device.
3. Time becomes irrelevant in 4 dimensional games. Or are you referring to its length on your own, personal timeline? In that case, probably as long as MI2 or something. When you've finished, you can go and become Merlin, if some other young whippersnapper hasn't got there first. Of course, the beauty of being free to travel through time is that you can always be there first, but that's neither here nor there.
4. It will be rank. Other, more popular games such as "Dinosaur Killer" and "Escape From The Psychic Circus XXXVIII" will eclipse it.
5. It will feature an entirely new cast consisting mainly of clowns from the aforementioned Psychic Circus, as by the time of MI5's production, the Circus franchise will be so mind-bogglingly profitable that LucasArts will pay tens of thousands of Yoricks (the only stable currency of the 29th century) to use them as special guest stars. They will then send the game back in time to the 21st century, where they will pre-empt the creation of the Psychic Circus and take all the profits for themselves, creating a paradox in the transdimensional time stream, but I don't think that will matter much.
6. The new bad guys will be caricatures of popular political figures of the 29th century, such as John "the Planet Eater" Morack and Paul "whoops I blew up the solar system by blowing my friend's trumpet too loudly, and no that isn't a euphemism" O'Connor. Unfortunately, LucasArts Of The Future LLC will fail to anticipate that their superb parody will make no sense in the 21st century, and so the game won't seem very funny at all. It will, in fact, be about as funny as "The Harbour" on a good day.
7. LeChuck will not participate, because LucasArts Of The Future LLC will have forgotten to add him in, what with their clever political satire and all. LeChuck has no place in the future anyway.
8. Elaine will also not participate. In the future, women will take over the planet (and indeed the universe, excluding the Andromeda galaxy which, as you know, is full of belligerent aliens) and enslave all men. Thankfully for men everywhere, this won't last long and all women will be destroyed in the Great Uprising of 2769, being replaced instead by androids who enjoy doing the washing up and never moan. By the 29th century, women will be nothing but a memory, and a bad memory at that. They will be banned from all forms of family entertainment, appearing only in horror films and low-quality sitcoms.
9. He will be very old. Fortunately, he is a fictional character, so he will not suffer any of the more unpleasant side-effects of getting old, such as death.
10. The voice of Guybrush will be 'done' by none other than Grand High Ruler of the Universe, Polluxious Grandous Deus Ex Machina XCLXXVII, the only computer capable of running the galaxy and voice acting simultaneously. It will be stupendously well done. The dramatic comments will hold audiences spellbound, and any sad moments LucasArts of the Future LLC might want to throw in will cause grown men to break down in tears at the first syllable. The effect of the voice acting on grown women, of course, won't be considered by the LucasArts of the Future LLC team, as women won't exist in the future. This could open LucasArts of the Future LLC up to litigation if it turns out that the Polluxious Grandous Deus Ex Machina XCLXXVII's voice acting causes 21st century women to (for example) come out in a rather nasty rash. Some commentators have suggested that this is part of the master plan, as such legal action would give Lucas Legal Of The Future something to do other than picking the small, defenceless fan-game sector, which will become a multi million-Squindar (the only stable currency of the mid-21st century) industry by approximately 2052. A Squindar is worth approximately as much as a grain of sand with anthrax spores all over it.
Do you doubt this? Do not doubt, child. It is the future. I have seen it.