As her resurrection drew near, her grin grew deeper - and vowed for her vengeance
This implies:
1) The person is not alive enough to be able to be resurrected
2) Somebody from Gensokyo is responsible for this (and it probably didn't happen in the outside world... but we can't rule that out)
OR the person wanted revenge for something before her death, but died before she could manage it
3) Ability to grin
4?) Not male? (Is the original japanese text gender specific?) edit while writing: somebody posted about this meanwhile. not gender specific.
Possibilities:
New character based on some folk tale or ancient japanese history (Undead Giant Enemy Crab loli?) as usual.
Mima Magia
Youki Konpaku, if someone killed... err, half-killed him.
(I also like this theory, but he's male too) Not gender specific, yay!
Someone who was unjustly beaten by Reimu/Marisa (/Sanae?/Youmu?) and died afterwards (not necesarilly a result of the beating though!) (Shinki, Tokiko, some random PC98 unnamed midboss thingy, pretty much everyone with enough speculation, as you can see below:)
Kurumi (assuming she is not alive enough to be able to be ressurected)
Yuki might want revenge if she heard Mai
YuugenMagan (who wouldn't want revenge for being poked in the eyes!)
Impossibilities:
Myouren Hijiri (natural death if i'm not mistaken)
(also he's male, so unless the japanese text wasn't gender specific, he's not the boss we are searching for. Move along.) Not gender specific
Remilia/Sakuya/whoever "Septette for the dead princess" refers to
Saigyou Ayakashi and/or unconsciously Yuyuko (i like this theory, but sadly i don't think Saigyou Ayakashi can grin...)
Yuka Kazami (probably still alive, also had her chance to have revenge in Touhou 9)
By the way, i think that if the person being resurrected is not the same person as the one resurrecting her, then stage 6 boss is resurrecter and EX is resurrectee (does this word exist?)