There is a pattern to Fairy tail's villians every team Natsu member has been related to a villian in the manga by blood Lucy-Jude heartfilea (hired phantom lord),carla-the queen(the entire race is kind of tied into this so this can count for happy too) in Edolas, Natsu-Zeref(first intorduced in Tenrou,Gray-silver, Erza-Ireen, Wendy-. Fallowing this pattern Wendy should have ties to a substantial vilian next major arc, and there is only one vilian left Acnologia.
This probably seems crazy but Sky dragon magic is the only slayer magic that we have seen to deal with souls (Milky Way). Irene doesn't seem to have used DS magic to ripped Wendy's soul out and Acnologia is stated have used his dragon slayer magic to extract them( what Wendy did with Irene even if she didn't use DS magic to do it?), his dragon form and grandie's original design have a lot in common, and his human for has similar hair to Wendy.
Besides that there is Erza and Wendy's bond being central to the arc have they been separated and the flow of recent battles Acnologia vs Irene, Irene vs Erza , Irene vs Erza and Wendy, Erza vs Wendy(black), Wendy vs Irene, Irene vs erza and wendy, and now we have Acnologia vs Wendy and Erza. there is also a striking similarity between these panels
http://www.mangahere.co/manga/fairy_tail/v38/c516/6.html http://www.mangahere.co/manga/fairy_tail/v38/c470/19.html http://www.mangahere.co/manga/fairy_tail/v38/c377/3.html http://www.mangahere.co/manga/fairy_tail/v38/c523/19.html
Acnologia was originally on the side of co-existence,but went insane from the side effects of Dragon Slayer magic if Grandeey taught her farther the magic that caused Wendy to be abandoned she would be obligated to take her in, and would be cautious about teaching the kid attack magic. There is also the question of why would Wendy need to be able to detach enchantments, Dues Zero played too central a role not be used again if Acnologia was a sky dragon slay that infused other powers then there would be a point to it.
Wendy's ability to activate DF at will may be odd to most but to me what's odder still is the lack of scales. To me the scale illustrate an encroaching nature of the side effects of the magic the absence of them and the presence of treats controlled by genes is something that leads me to think she is activating ties that go deeper than just her magic. Every other transformation they are present at lest on the face everything else has always been secondary, and their presence has been fairly extensive since the GMG except in one case that is where features normally untouched by such transformations change to completely.
http://www.mangahere.co/manga/fairy_tail/v12/c098/6.html http://www.mangahere.co/manga/fairy_tail/v19/c160/2.html http://www.mangahere.co/manga/fairy_tail/v38/c411/13.html http://www.mangahere.co/manga/fairy_tail/v26/c295/2.html http://www.mangahere.co/manga/fairy_tail/v38/c511/21.html http://www.mangahere.co/manga/fairy_tail/v38/c487/23.html http://www.mangahere.co/manga/fairy_tail/v38/c487/26.html http://www.mangahere.co/manga/fairy_tail/v38/c377/3.html It would different if it was just once but every other case?
Grandeeny has feathers instead of scales but the protrusions do not mimic either design in placement they more closely mimic Wendy's original dress which she has never worn again. if those feathers are the equivalent of the scales then they should be present each time she transforms
http://www.mangahere.co/manga/fairy_tail/v38/c432/19.html http://www.mangahere.co/manga/fairy_tail/v38/c432/20.html http://www.mangahere.co/manga/fairy_tail/v38/c516/8.html http://www.mangahere.co/manga/fairy_tail/v38/c483/16.html they aren't there entire Irene fight. The hair is an extreme unique to Wendy and especially the eye color change doesn't make sense at all since even takeover hardly ever touches those and even in those cases it is less dramatic. The hair is a throw back to Porlyusica but the pink eyes have no base but the bigger point her is these are things that are controlled by one's genes and untouched by any other slayer magic and only rare cases of take over magic and even then never the eyes. She was only five when sent back that is young enough to have had a early slayer as a parent. If her activating it at will was linked to genetics then it would explain the lack of the encroaching nature (scales) and the presence of the genetically controlled treats that go untouched by the magic in every other case while also going some what explain her learning rate with the magic. Also while this page
http://www.mangahere.co/manga/fairy_tail/v26/c295/4.html makes it seem that it is exclusive to the third gen this page
http://www.mangahere.co/manga/fairy_tail/v38/c415/8.html suggest that the secret to free will transform could not even be required to have the lacamum and a dragon's lifeforce could be contain in the blood verse Irene and Acnologia or in sting and rouge's a lacamum but Wendy has never killed a dragon and shouldn't have had her dragonfication advanced so it is an oddity that does not fit a pattern unless her's comes from blood of different kind.
So this what makes me think Wendy is the daughter of Acnologia what do you guys think? Sorry for the extremely long post.