I believe that will be the last area of dream world. there's not much pokémon to appear on DW, Spelon, pamtre, nomel and rabuta are the last useless berries to appear, probably the other berries will appear as gift on tournaments or on special areas just like Pokemon Cafe forest.
Pokémon families that didn't appear on regular dream world:
Bulbasaur, Ivysaur, Venusaur (Chlorophyll)
Charmander, Charmeleon, Charizard (Solar Power)
Squirtle, Wartortle, Blastoise (Rain Dish)
Weedle (run Away), Beedrill (sniper)
Ekans, Arbok (Unnerve)
Pichu, Pikachu, Raichu (lightning Rod)
Paras, Parasect (Damp)
Venonat (Run away), Venomoth (Wonder Skin)
Grimer, Muk (poison Touch)
Happiny (friend Guard), Chansey (Healer), Blissey (Healer)
Pinsir (Moxie)
Ditto (Imposter)
Eevee (Anticipation), Vaporeon (Hydration), Jolteon (Quick Feet), Flareon (Guts), Espeon (Magic Bounce), Umbreon (Inner Focus), Leafeon (Chlorophyll), Glaceon (Ice body)
Porygon, Porygon2, Porygon-Z (Analytic)
Munchlax, Snorlax (Gluttony)
Chikorita, Bayleef, Meganium (Leaf Guard)
Cyndaquil, Quilava, Typhlosion (Flash Fire)
Totodile, Croconaw, Feraligatr (sheer Force)
Togepi, Togetic, Togekiss (Super Luck)
Aipom, Ambipom (Skill Link)
Unown
Pineco, Forretress (Overcoat)
Heracross (Moxie)
Treecko, Grovyle, Sceptile (Unburden)
Torchic, Combusken, Blaziken (speed Boost)
Mudkip, Marshtomp, Swampert ( Damp)
Wurmple (Run Away), Beautifly (Rivalry), Dustox (Compoundeyes)
Seedot, Nuzleaf, Shiftry (Pickpocket)
Shroomish (Quick Feet), Breloom (Technician)
Slakoth, Vigoroth, Slaking
Nincada (Run Away), Ninjask (Infiltrator), Shedinja
Plusle
Minum
Budew (Leaf Guard), Roselia (leaf guard), Roserade (Technician)
Gulpin, Swalot (Gluttony)
Zangoose (Toxic Boost)
Seviper (Infiltrator)
Kecleon
Turtwig, Grotle, Torterra (Shell Armor)
Chinchar, Monferno, Infernape (Iron Fist)
Piplup, Prinplup, Empoleon (Defiant)
Kricketot (run away), Kricketune (Technician)
Combee (Hustle), Vespiqueen (Unnerve)
Cherubi, Cherrim
Riolu (Prankster), Lucario (Justified)
Carnivine
I won't do the list about the unova pokemon,because I believe that those pokemon are a different case, Dream world main purpose is to make people have access to old pokemon with new abilities.
Game freaks could do a special area with all unowns, I know that it's useless for battle purposes, but they could do a special minigame and with some useful items.