I've had a quick test now for performance with low alpha coverage (high overdraw) and been pretty happy with the performance impact. I've also had a go at seasonal variation for growing and shedding leaves and changing colour. All of that seems to be working fairly nicely though it has become clear that if I want my trees to blossom I will need another texture.
I've moved on to the trunk model. I expect I'll need a few size variations, but I don't really want to add too many branches to them, since I can have more variation with relatively little work by having branch models which I can intersect with the main trunk geometry.
My first stab at the trunk came out okay, but a little bland.
I took a look at some more reference, and really liked this tree's style.
I decided to work in mudbox to speedily try some more confident shapes.
Since this is a fantasy environment I don't really mind exagerrating some of teh features of trees to look a bit more interesting.
I then retopologised in Max. This process is still pretty tedious - I expect I could do a faster job but I'm a real perfectionist when it comes to topology so it takes a long while.
Clearly there are a few quite distinctive features, but I'm hoping that by rotating and scaling the trunks and by intersecting them through the floor at different heights, I expect the player won't notice every large tree has the same trunk... particularly given their branches will be in different positions.