I don't want to tell Square Enix how to do their job or anything (They really have spared no expanse in the marketing of FFXV) but I was thinking that we, in our role as consumers can tell them what would appeal to us. I really want this game to succeed.
Firstly, I think there needs to be a gameplay trailer laying out the various things you can see and do in Final Fantasy XV(preferably the last trailer). To see what I mean, look at the gameplay trailers for Grand Theft Auto V and The Witcher 3: Wild Hunt.
There have been many complaints about how there isn't much to do and all the areas look the same. I don't think the reason is that FFXV is lacking in these areas, I think the problem is that a lot of the trailers have been focusing on single aspects of the game. A lot of people don't see all the trailers. They are left with an incomplete picture of FFXV. Bringing them together would better show the variety in FFXV.
In the trailer, emphasis should be put on different areas(people have gotten used to Duscae, maybe show us a bit more of the snowy train area?), the various monsters(people have been pointing out how the FFXV monsters really look like living beings), combos(to demonstrate how the party members work together), summons(people were very impressed by Rahmuh, it got the series new fans) variety of activities(show us more monster hunting, camping, chocobo riding, fishing, boss battles and random events such as people found on the side of the roads). Also, unlike in the examples I have given, emphasize the the story in this trailer too because that is very important to most fans. Including all this will leave newcomers with a better picture of the game.
Another important thing would be to do an AMA on Reddit's /r/IAMA. Reddit is the 9th most visited site in the USA and 33rd in the whole world. Anything that gains popularity there will soon spread to all over the internet. Mr Tabata already did an AMA but it was about Final Fantasy Type 0 so this one should focus on FFXV.
To make the most of the AMA, it should be started between 8 - 9 AM EST on a weekday as this is when most reddit users are active and the post has a higher chance of being upvoted. This should ideally be done really close to release (Like the No Man's Sky AMA) to really catch people's attention.
So, can you guys think of anything else?