Is this still under discussion or all decided up now?
I like the freshwater plan. The lake could have a more complicated shoreline, with inlets and peninnsulas and a larger bay, in addition to one or more large islands.
I suggest some forests to be old growth but mostly under the Sleep of Yavanna, dormant. The living (awake) woods could be more open woodlands. I think there should also be plenty of more open land outside the woodlands. JRRT mentions both fir and birch and I don't like the idea of a subtropical climate or landscape -- the first map from the first page of the thread is based on the pre-LotR Ambarkanta, but the map concepts in that text seem to have been later changed by JRRT -- it isn't clear at all what replaced them because he never drew another map on that scale, but he switched Hildorien from equatorial East Asia to Mesopotamia (well north of the equator), and Cuivienen was supposed to be far enough north and east of Hildorien that the Men awoke "in regions which neither the Eldar nor the Avari have known". It specifically can't be the Mediterranean, since that is probably the basis for the Bay of Belfalas. At the same time Cuivienen can't be so far north that there's no room for Utumno to be even farther north, where the rays of Illuin were dim.
(The second map in the first post is so goofy I don't know what it's supposed to be.)
In the Cuivienyarna (a post-LotR legend in The War of the Jewels, History of Middle-Earth vol. 11), the Elves are described as awakening on a green sward in a dell, in a larger dell, in "a birch grove by a stream", "a sweet-smelling firwood on a hill-side", and beside a waterfall coming down from a cliff into a lake. They lived beside a lake rather than saltwater sea, both in Cuivienyarna and the companion text Quendi and Eldar, so that fits the freshwater lake decision above.
In Quendi and Eldar "...before the Separation [the Nelyar] never moved far from the lake and waterfall of Cuivienen..." Meanwhile, I vaguely remember Noldor liking hills and Vanyar liking flat plains, but maybe that's only where they lived in Aman. If anyone is going to build rafts and canoes and reed-boats, it should be Nelyar/Lindai. If anyone lives up in trees, I think it ought to be the Tatyar (though they could also build stone huts) or Vanyar. Flets are supposed to be an innovation of Lothlorien, so maybe whoever lived in them before stopped living in them later, or were all Avari.
I get the impression that Elves at Cuivienen did not cut down trees or hunt beasts or birds, because the Laiquendi got quite upset when mortals did those things in Ossiriand. I'm guessing they ate fish (sturgeon!), plants, and mushrooms, and perhaps things like frogs and beetle larvae that the Eldar may later turn their noses up at*. For clothing I think they would not wear leather, but plant fibers (flax, bark-cloth) and weave or felt hairs shed by large animals (like woolly Ice Age megafauna...). I imagine the Elves are friends with animals (especially if they don't hunt) and can just walk around among herds of them picking up shed fur.
I can't recall whether they were supposed to know anything about smelting metal. I thought the Valar supplied the Eldar with or taught them to make metal weapons on the March, for self-defense. It would be reasonable to thoughtfully make those available to the Avari who stay behind in the monster-haunted wilderness, too.
*I also have an idea that, because there is no rot in Aman, there are no mushrooms, so the Noldor born in Aman are disgusted to learn Sindar and Nandor eat these things -- especially when they learn that most fungus is poisonous. It could be funny, or just show the snobbery of many Noldor towards the Moriquendi.
(Even some of the Valar are elder siblings to other Valar, despite being created before time itself.) 
