Return of the Heroes
So what is a Return of the Heroes? It's the game to get some of the old group back together as well as invite new ones. First of all you need to have everyone recollect their favorite characters or a combination of their favorite characters. Give them 6050 XP and start (re)making characters. You could also make the players the children of their favorite characters! Tell them to not buy equipment as you the DM will make an equipment list for them. Give them a minor few magic items and such.
Next start with the adventure. Try and reuse or rehash old villains (or the children of the villains). This automatically makes the party join together and be ready to fight. Remake and expand you dungeons of old. Really go all out on this.
Ronvug that evil wizard is now a Lich and he has many minions out there! The players will have to get more powerful to defeat him!
Deep in the the wilderness in the middle of a caravan attack is how it needs to start and the goal is the Tavern in the small walled keep or town!
Where were the characters coming from? Were they at war or on a quest that required them to all separate and years later come back together?
Now they have all received a summons to return to their old meeting place. They have heard dark news that their old enemy (or his offspring) have returned.
It is a time for the Return of the Heroes!