1st level
Human Fighter [Dueling]
Human Cleric [Light Domain]
Human Wizard
Elf Fighter [Archery]
Dwarf Rogue
Halfling Rogue
At 5th level
Human Fighter is a Battlemaster [Commander's Strike, Menacing Attack, Riposte].
[Human Light Cleric died at 2nd level against some ghouls. First casualty of the game. His sacrifice lives on in song.]
Replaced by...Human Cleric 2 [War Domain]...currently 3rd level.
Human Wizard is a Transmuter.
Elf Fighter is an Eldritch Knight. Take the Magic Initiate Feat for more spell power.
[Dwarf Rogue died at 2nd level fleeing from some ghouls]
Replaced by...Half-Elf Ranger...currently 3rd level.
Halfling Rogue is a Thief.
At 10th level
Human Battlemaster Fighter is a shell of his former self because his lover, the half-elf ranger, died at level 7. Working towards raising an army to wage war against the known Orc tribes/kingdoms [Commander's Strike, Menacing Attack, Riposte, Disarming Attack, Lunging Attack, Rally]
Human War Cleric is 8th level.
[Human Transmuter Wizard slain by demon]
Replaced by a Dragonborn Dragon Sorcerer...cuz...dragons. 7th level.
Elf Eldritch Knight has been fulfilling the role of "party mage", having inherited the Transmuter's spellbook, and has taken 2 levels of Wizard to get to the transmuter tradition to use it to its fullest. So, currently Eldritch Knight 8/Wizard 2. But still likes using his arrows when he can. Doesn't trust the sorcerer very much.
[Half-Elf Ranger slain by orc ambush.]
Replaced with Half-Elf Druid...7th level.
[Halfling Thief Rogue slain by poisoned needle trap.]
Replaced with Halfling Ranger [Hunter]. Currently level 8.
New player showed up around level 8...so add Half-Elf Paladin [Ancients]...sister and "bodyguard" of the new half-elf druid. Based on when they entered the game, is currently at level 7.
So, "by 15th level", looks something like this:
Human Battlemaster Fighter, 15th level. Ready to retire, having decimated the orcish lands and banished the influence of Gruumsh from the world for a thousand years. Ready to settled down in the party's stronghold home base with the half-elf paladin.
Elf Eldritch Knight/Wizard, 10/5.
Dragonborn Dragon Sorcerer, 12th.
Human War Cleric, 13th.
[Half-Elf Moon Druid slain by demonic hordes of Gruumsh. Over the course of 15 levels the player has still not learned how to keep their PC alive. End of campaign. No replacement.]
Half-Elf Ancients Paladin, 12th. Devastated by her brother's death, found solace in the Battlemaster Fighter and now the two are ready to "retire" from adventuring, rule their lands nad start a family.
Halfling Hunter Ranger, 12th.
EDIT: I don't know if this is a "dream team" per se. But sounds like it would be a pretty cool/fun group and game to be in...and expresses some fairly common, to my experience, situations and issues within any given group.