Psycho Busters fits perfect... the main char is a normal guy who meets extraordinary people and then later on....chapters later gains power himself. ^^

Berserk?
Don't know what you mean about tragedy... It's like very few mangas can stay away from tragedy and keep being interesting, none of them with people becoming super natural...

But if you see the general plot, and overlook the dark details, it's like Berserk ain't much tragic than Naruto... Pretty similar in some ways, you know... Friends that become foes, death of important characters... A destiny of misery for the main character, and still he fights for the same reasons any human wold fight, humanity, friends, power and revenge.

Wow, no i'm depressed... don't read Berserk...

Inugami fits but not much occurs with the whole transformation.
Psycho Busters doesn't fit because you don't become a psychic just your potential is unleashed as one.
Try to avoid Sci-Fi, think more Supernatural, Horror, or Fantasy.
Please try to find more with Romance if possible.
And Berserk sounds too depressing for me too read.

________________
All choices are made in fear,
What you fear more is the only difference.
Positive decisions are made in fear negative consequences,
Negitive decisions are made in fear of positive decisions not giving the outcome you want.
Embarrassment is the fear of people knowing your real thoughts.
Greed is the fear of having too little.

Demonblaze0's personal thoughts

it's been a long time since this thread was commented on so maybe new manga that fit the subject have been made
so i'm reviving this thread

________________
All choices are made in fear,
What you fear more is the only difference.
Positive decisions are made in fear negative consequences,
Negitive decisions are made in fear of positive decisions not giving the outcome you want.
Embarrassment is the fear of people knowing your real thoughts.
Greed is the fear of having too little.

Demonblaze0's personal thoughts

Firstly, do you only want manga? There's a good example in a novel series.
http://en.wikipedia.org/wiki/The_Keys_to_the_Kingdom
Dying human boy slowly becomes more divine and godly with each Key of Authority over creation he picks up.

As for manga, I think the best one that is recent to your revival of this question that includes romance and fantasy and slowly turning into a hazard over the course of the series would be Witch Craft Works.
http://www.mangaupdates.com/series.html?id=52253

I'll reiterate about Berserk, it's extremely dark but their recommendation was spot on.

There are examples of a main character becoming an immortal after being on the verge of death in Nanatsu no Taizai.
http://www.mangaupdates.com/series.html?id=82239

The entire story for Gokukoku no Brynhildr is about humans being turned into scientific experiment powerhouses for otherworldly aliens. Primarily about romance motivating the MC to do crazy things just to survive.
http://www.mangaupdates.com/series.html?id=72810

As with above, Deadman Wonderland is a quick read but the MC gets his supernatural ability at the start.

If you want romance and dealing with the transformation into something not human.
Shishunki no Iron Maiden http://www.mangaupdates.com/series.html?id=86190

And if you want an unorthodox recommendation under romance and fantasy and becoming a demon for love, go read the Madoka Magica manga.

Tokyo Ghoul
Sengoku Youko Happens to a few characters, one of which becomes MC in 2d arc.
Ajin
Rurou no Tabi
Itsuka Tenma no Kuro Usagi
Kurozuka
Blade of the Immortal

These have the tragedy tag, but honestly you should still check them :
Apocalypse No Toride Happens kinda late in the story though.
Shiki
Priest
Shingeki no Kyojin

Imawabi no Dakini seems to be going that way

Zetman
Yu Yu Hakusho
Berserk
Bleach
Shingeki no Kyojin

Tokyo Ghoul fits very well, read to current scan after being recommended and I really like it
I am going to try some of those that have been recommended
Also I have already posted that I do not want to read Berserk so please stop recommending it

________________
All choices are made in fear,
What you fear more is the only difference.
Positive decisions are made in fear negative consequences,
Negitive decisions are made in fear of positive decisions not giving the outcome you want.
Embarrassment is the fear of people knowing your real thoughts.
Greed is the fear of having too little.

Demonblaze0's personal thoughts

This makes me think of Dance in the Vampire Bund -- romance between a werewolf and a vampire.

nope not looking for interspecies romance but transformation from human to something supernatural
romance not required but prefered

________________
All choices are made in fear,
What you fear more is the only difference.
Positive decisions are made in fear negative consequences,
Negitive decisions are made in fear of positive decisions not giving the outcome you want.
Embarrassment is the fear of people knowing your real thoughts.
Greed is the fear of having too little.

Demonblaze0's personal thoughts

Well this isn't even categorized as supernatural but..... KUROKO NO BASKET definitely going overboard with the special abilities, emperor's eye, the second door of zone and what not.

________________

I started reading (listening) to this after seeing your recommendation... and I just have to say I'm getting really annoyed, lol. I'm on the third book and it seems to me the story is more about him NOT becoming a supernatural being (denizen) over the course of the story. Every time he gains a key/power he immediately gives it up! Then he's immediately set upon by enemies and has a really hard time as powerless as he is. What a moron. Really annoying, I can never sympathize with characters "just wanting to be normal again". So tell me. How appropriate is your recomendation here? Does he ever actually keep power/become immortal? Because if not I really don't want to find out at the end of the series.

Tokyo Ghoul also has an anime too that's just coming out now.

There is also a Becoming Something Inhuman tag, so that's worth checking out.