There are more untagged hidden characters in this and I'm too lazy to search for them and research their names. There's one in the lake and the one getting burned by the dragon is too small for me to identify.
1st row Vegeta · Trunks · Nagato 2nd row Konata · Tsukasa · (couldn't read the 3rd kanji, the first 2 says Sekai, but it could be other reading) 3rd row Bubble Slime · Slime 4th row Yokohama Tire · Chimera · RODO (could be Lord) last 抵大乙 (i hv no idea what this is)
since the obvious character clearly visible aren't in it, could be the names of hidden chars
Vegeta: Trees on the left below the mountain SSJ Son Goku (or maybe Trunks?): To the right, Half mountains half lake Nagato: fighting the dragon on the upper-right (ahh, i see now) Konata: Ahoge only Tsukasa: Left clouds
Can't find the Yokohama Tire (logo?)
Don't know the others that you mentioned and still can't figure out what/who is that on the center of the lake.
1. I see that you don't put notes so others can have the joy of searching as well 2. is there any significance of the tree larger than mountains on the back there? 3. there has to be a larger pic out there, this one is desktop resolution...
世界樹 solves two mysteries in one. I think at the bottom of the sign it says 拡大乙, which could be Hiroshi Ootsu if it weren't in the wrong order. Roto might be related to http://en.wikipedia.org/wiki/Emblem_of_Roto.
I'm pretty late to this, but I thought I'd point out Bubble Slime being a leaf in the middle of the screen, and what I think to be Slime, which is that darker area of grass, the oddly distorted trees and the, what looks to be, eyes of the slime on the lake.
Vegeta - Trunks - Nagato
Konata - Tsukasa - World Tree
Bubble Slime - Slime
Yokohama Tire - Chimera - Roto