While I would want some more forestmen and the sort, I really want some elves. So, Elves is what I voted for.
I hope you don't mind, but I have some Counter-Opinions.
"4. Forestmen sets also have vehicles (carts, wagons, boats, etc) which are very useful in building your castle world. "
Who's to say the elves won't get a wheeled vehicle and/or boat? A good majority of the current 'Fantasy Era' Castle sets include a vehicle of some sort.
"5. Elves have interesting structures, but Forestmen have cool hideouts with traps, secret doors, etc. "
Well, as I see them, elves are very crafty and would probably try to protect their forest, so I could see some new sort of Camoflauged outpost and Booby traps in with the new elf sets.
I mean just look at how well Lego did Indiana Jones, I'm sure they would be able to make some sort of 'trap' suitable for a forest.
"6. Forestmen need plumes. "
Our current knight helmets don't have plumes, yet it is kind of something that is expected. Lego might just release a new hat or two for the Forestmen, but no plume. With elves we might get a new hat, maybe even new hairpieces. Nothing is definite though.
In the future, a ressurection of Forestmen/Dark forest would be nice, but right now is Fantasy's time.
(Sorry that had to be so lengthy)