Yeah, lacrima. My bad :P
I agree with you about Erza. She used to be one of my favourite characters. And i never disliked the fact that she had more pof than anybody else. But recently, especially against Kyouka and the Historias, this has been quite disappointing.
Mavis actually said that Cana had Gildarts' blood ruining through her veins and that Cana had an immense latent magical power.
http://mangafox.me/manga/fairy_tail/v34/c285/20.html
http://mangafox.me/manga/fairy_tail/v34/c285/21.html
I agree with the lacrima thing, as i said above. It was my mistake. I definitely don't think this is a coincidence though. He could have had any element, why would Ivan chose Lightning? Maybe Laxus' potential was friendly towards that particular element because of blood. I dont even know if my sentence makes any sense at all, and the theory itself might be far fecthed i admit. =)
However, Laxus has been said to have inherited Makarov's magical power. This has been said by Freed, Hades and probably some more that i don't recall. I agree that with training anyone can become strong, but before even becoming a beast through training, Laxus was always respected for his huge magical power coming from Makarov, a thing which annoyed him to the point where he wanted to prove he had surpassed grand-father. Therefore, battle of FT, and then when he stated to Jura that he was tired of people bring the blood relation on the table everytime.
http://mangafox.me/manga/fairy_tail/v15/c125/14.html
EDIT: forgot the last part of your post.
Well Romeo, i might have remembered wrongly. I thought he already had fire magic (from Macao), and started training that to become stronger, with Totomaru (idc about his name, the 4 elements - fire guy from Phantom Lord). If he didn't have that type of magic to begin with, then my bad.
EDIT 2:
But anyway, Lucy is a celestial mage as well as her mother, Ultear knows Ul's ice magic, but we could say that Brandish isn't a celestial mage contrary to her mother and has a gigantic power which probably (we don't know though) doesn't come from Grammi.
There are a lot of examples and counter-examples.
I seem to recall something from the anime, that i could't find in the manga. When we have a flashback of young Cana being shy around Gildarts, and at some point she wants to know when he'll come back. Someone tells her "eh, Card magic would allow you to predict that" Cana says "wtf im not a mage, how could i know Card magic", the person answers "you can learn it, magic comes from the heart, blablabla"