I think the 16 Retro Tracks will be:
SNES Donut Plains 2
SNES Vanilla Lake 1
SNES Mario Circuit 4
GBA Riverside Park
GBA Snow Land
GBA Ribbon Road
N64 Yoshi Valley
N64 Royal Raceway
GCN Mushroom City
GCN Wario Colosseum
DS Bowser's Castle
DS Tick-Tock Clock
Wii Daisy Circuit
Wii Dry Dry Ruins
MK7 Mario Circuit
MK7 DK Jungle
I kept in mind that they'll automatically choose at least one snow, desert, mario and bowser castle tracks

The complete list of unused tracks are below, to make it easier for you to choose from:
SNES:
Ghost Valley 1
Bowser Castle 1
Choco Island 1
Donut Plains 2
Bowser Castle 2
Koopa Beach 1
Vanilla Lake 1
Bowser Castle 3
Mario Circuit 4
Donut Plains 3
Ghost Valley 3
Vanilla Lake 2
GBA:
Riverside Park
Mario Circuit
Boo Lake
Cheese Land
Cheep-Cheep Island
Sunset Wilds
Snow Land
Ribbon Road
Yoshi Desert
Lakeside Park
Broken Pier
Bowser Castle 4
Rainbow Road
GC:
Dry Dry Desert
Sherbet Land
Mushroom City
Wario Colosseum
Bowser Castle
Rainbow Road
DS:
Figure-8 Circuit
Cheep Cheep Beach
Tick-Tock Clock
Mario Circuit
Shroom Ridge
Wario Stadium
Bowser Castle
Rainbow Road
64:
Toad’s Turnpike
Wario Stadium
Yoshi Valley
Royal Raceway
Rainbow Road
Wii:
Luigi Circuit
Moo Moo Meadows
Toad’s Factory
Mario Circuit
DK Snowboard Cross
Wario’s Gold Mine
Daisy Circuit
Grumble Volcano
Dry Dry Ruins
Moonview Highway
Bowser’s Castle
Rainbow Road
MK7:
Toad Circuit
Daisy Hills
Cheep Cheep Cape
Shy Guy Bazaar
Wuhu Island Loop
Mario Circuit
Melody Motorway
Alpine Pass
Piranha Plant Pipeway
Wario’s Galleon
Koopa City
Wuhu Mountain Loop
DK Jungle
Rosalina’s Ice Land
Bowser’s Castle
Rainbow Road