I have several adventures that I was considering running for 5th edition. I was considering Red Hand of Doom or Horde of the Dragon Queen/Rise of Tiamat. I think I finally decided on a combination of both with some original content thrown in.

Here is my proposal.

The players are a D&D group that get summoned to a Fantasy Realm that is in need of heroes. However, they do not appear as themselves but arrive in the game world as their characters. The GM is now a powerful Warlord controlling the enemies that threaten the land. Can the players find a way home? Will they want to go home?

Would anyone be interested in this? Would you want to create your own characters or have yourselves take 'pregenerated' characters. Would you prefer to know what characters you will become of have a completely random selection.

How do you want to deal with the transferral. Do players keep their own personality or gain the personality of their characters... or something in between.

Does anyone have any advice or suggestions for a game like this?

 Sounds like the novel series called Guardians of the Flame. Just the flow you decided
 for your game and as you posted.

 It sounds like fun. I am familiar somewhat with 5th, much more with 3.5 but have been wanting to give 5th a solid play test. No problem if you don't want newbies so to speak in your game. Either way, go for it.

How important is it for the characters to have come from a RL DnD group? If it is important, what is the backstory for this RL DnD group? How much do these players know each other? Etc, etc. If that doesn't really come into play, then you may want to consider leaving that out.

Loved me the Guardians of the Flame.  This is very interesting to me.  I think keeping personality the same is good, but of course letting the character adapt to the new world/abilities is a lot of the fun.

I'd vote they keep their original personalities but are influenced by the personality of their character to some degree (at their discretion).

It's not completely important and 'native' characters could be allowed. I just thought it might be a fun twist. I was basing it more on the new 'Jumanji' instead of 'Guardians of the Flame'.

I was thinking that the group is reuniting after a long hiatus (several years). Some might have reasons to stay (illness, unhappy with their current life, etc). Some have reason to return. (Have family waiting... the jock doesn't want to be a shocking girl... the pretty girl doesn't want to be a half orc barbarian, or maybe she likes being strong... etc.)

It does sound pretty interesting.

What are you thinking for starting level and such? Would the players be starting a new game? Or would they come into a game they’d been playing for a while and had put on hiatus?

Started up the game. See the players wanted section.