Something I noted while looking up information for the discussion on Cracked Sand and sandstone, is the fact that in nature sandstone, and sand in general, come in a variety of colours. Upcoming in a future vanilla update is the ability to dye armour (at least leather armour). Add to that the presence of Red Rock and Autumn Woods biomes in ExtrabiomesXL already, and I'm beginning to think that adding more colour to the world would be a great goal, in line with the already gorgeous improvements to the landscape.

It would be nice to see white and black sand beaches, both of which are quite striking in the real world. While red sandstone might be a bit redundant with Red Rock, I think it's worth mentioning too, as it would allow the addition of painted deserts (a natural phenomena in which bands of differently coloured sands form in a desert), and striped or "rainbow" sandstone could lead to some interesting effects. There's also some nice looking orange stone in the world, which could be a good compliment to Red Rock.

Next of all, one of the things that led me to run Trees++ alongside ExtrabiomesXL, even though I don't care for everything in said mod (I could do without crystal tools, and some of the naming conventions suck) was the vibrant purple and pink of Jaceranda and Cherry trees. While I would hate to steal their thunder, this is one area where EBXL could have a bit more to look at. I will try, below, to list things that are not redundant with their mod (though simply working with their team on a fully compatible version, cleaning up some sloppiness, and getting an end product that can be treated as a proper addon might be more expedient).

• Prunus Virginiana 'Schubert', the Schubert choke cherry, has a very nice purple foliage. They grow in temperate regions with low to moderate moisture. As the foliage of the choke cherry reaches its purple hue in summer, they would be a good choice for a colourful plant that can exist outside of Autumn Woods zones.
• Acer Palmatum, Japanese maple, produces shades of orange and red in the spring. These colours are already present in Autumn Woods, but it suggests that trees with similar foliage could appear infrequently in other biomes (while I love birch, the Birch Woods could currently use something to break the monotony.)
• Rainbow Eucalyptus...this one has to be seen to be believed. http://www.amusingplanet.com/2011/10/rainbow-eucalyptusthe-most-colorful.html

The last is probably completely unsuitable, even as a very rare tree. People just wouldn't believe it. However, it's a good example of the startling degree of colour that nature can produce. This segues well into my next though, which is trees with naturally colourful wood.

• Peltogyne, purple heartwood, isn't so much a showy tree, but as you might guess from it's name is prized for the rich violet hued wood that comes from it. Which could make it of great interest to builders.
• Although represented in Trees++, it's worth noting that many redwoods do indeed have red wood.
• Ebony is a well-known black wood, for those looking for a truly dark colour.
• Finally, there is in fact the unimaginitively named "Yellowheart", with an expectedly golden colour.

@Rosethorns also suggested kauri trees.

Regarding trees++, I have spoken to the developer. While he wishes to be compatible with this mod, he does not wish close collaboration.

I like everyone of these ideas and I am thrilled to have your input. I too play with Trees++ as a complement to Extrabiomes because of the additional variety.

Some of the colorful cherry trees were in an addon to the original pre MC 1.2.5 mod. Due to a falling out, we had removed them because we wanted to honor that authors wishes, though we have always had plans of reimplementing some of those beautiful trees.

I have no issue with content overlap with Trees++, as long as we get to that point though originality. A great goal for this mod would be to meet all of a players terrain needs aesthetically speaking, not from a desire to compete with other mods, but from a desire to develop the best mod possible. I love Trees++, but it has always indicated a missing piece of this mod that I feel I have to install it alongside.

I have another small mod, Bunyan, that attempts to make the trees more interesting. It even introduces new types and colors of wood (Redwoods are redder.) The 3.x series of this mod will have some architecture improvements and merge Bunyan in as an optional part of the mod.

In developing Bunyan, I was attempting to add new and more interesting trees. I ran discovered very real limitations in myself: It is very difficult for me to change modes from "technically creative" to "artistically creative" and the latter is much more of an effort, so it becomes a frustration and seems burdensome. If I had (hint, hint) someone who put the artistic energy into speccing the trees, it would be a lot more efficient for me to implement them. I don't need highly structured code...just and artists thoughts and instructions on how they might place the blocks to make the tree look natural. This mod has high quality trees and because of the above reasons...my attempts often don't meet the bar.

Finally, in vanilla minecraft biome = humidity, temp, altitude range, grass type and tree type. This is fine when you only have 3 or 4 trees, but it doesn't really make sense when you have a dozen or two species. I am trying to explore ways to spawn groves of trees based on temp, humidity and altitude, without a direct mapping of biome to tree. (For instance, why not have a sakura grove spawn on the appropriate mountain side, rather than having to have a sakura grove biome.) In this way, we can accomplish terrain gen features like natural tree lines at tremendous altitudes, lush rainforests and jungles all organically. This, to me, is the holy grail.

In regard to the possibility of placing aesthetic features more naturally, it may be prudent to look at how features are placed, rather than how terrain is generated. For instance, villages seem to adjust to terrain, spawn in diverse biomes, and change composition depending on location. These are all things that a grove of trees should be able to do.

As to tree styles, I might see if I can mock-up something.

Haha! I've never actually seen that one before.

As it won't let me comment then close I am adding the info here:

By Annysia: Since FN has become or will become part of EBXL in v4 as well as many new trees...etc and since the comments about features will be mostly taken care of by the new terrain generator we will be using, I am closing this issue.

Most features that are placed adjust the terrain to themselves...which is why a village placed on a ravine has farms on 60 block high plateaus...but I get your point and you are right.