Ok, I definetly say to give Blind Guardian and Helloween another chance. First of with Blind Guardian - Nightfall In Middle-Earth did not do much for me at all when I first listened to it. It grew on me overtime but to this day it still is not one of my number 1 favorites by them but still a good album nonetheless. Listen to albums like Tales From The Twilight World, Somewhere Far Beyond, and Imaginations From The Otherside. All killer stuff. If you listen to that stuff and still don't dig em, then drop em, but atleast give those albums a shot.
As far as Helloween goes you listened to the Michael Kiske era which is their most well known and popular era. I love that stuff but if you didn't like it I suggest checking out the current Andi Ders era. Andi is a great frontman and has released many great albums with the band including Master Of The Rings, Time Of The Oath, Better Than Raw, The Dark Ride, and Rabbit Don't Come Easy. His vocals are a bit different from Kiske's and if you aren't into the happy helloween music definetly check out the dark and moody Dark Ride album. If you do like the Dark Ride album then you would love the band Masterplan. Masterplan is where Uli Kusch and Roland Grapow went to form after The Dark Ride from Helloween because the rest of the band Helloween wanted to go back to their happy helloween roots but these guys wanted to stay making dark and moody power metal which is awesome. I like both bands and can go either way.
As far as other bands go, there are many. I see you like Hammerfall which is awesome. Legacy Of Kings is my favorite by them. Iced Earth I see you are also into which is great. I just saw them this past Monday and they were incredible. It was my second time seeing them this year. Children Of Bodom is an important band to check out if you don't mind deathy vocals. The music is brilliant and I like the way the vocals accompany them. They are getting very popular in America so be on the look for them. Also Stratovarius is an very important band. They have released many essential power metal albums and aside from all the drama that band has been through the past few months I think they are gonna put things together and stay together. Check out Visions, Destiny, Episode, and Infinite from them. All great albums though they dont really have a bad one. Running Wild and Grave Digger are two other important bands to check out. They have been around since the beginning of power metal in the early 80's and are still running strong to this very day and making new material. As a matter a fact a few weeks ago I flew from NJ to OH to see Grave Digger's first ever show in America after being together nearly 25 years. It was a night ill never forget and they are one of heavy metals finest bands. So yeah, definetly look into Running Wild and Grave Digger as well as their German peers Sinner and Rage who have been there since the beginning as well.
Edguy is another important band. To me Edguy is the future Iron Maiden. At the rate their going they are definetly setting themselves up for this. Each album they release is better than the last and they don't have a bad one. Hellfire Club which just came out this year is easily my favorite album in years. Its just so good. Tobias sounds a bit reminicent of Bruce Dickinson though not a rip off of him. The band is just incredible and are also on the rise in America. Its bands like Edguy, Iced Earth and Blind Guardian that are the Maiden's, Priest's, and Dio's of the future.
Other great bands to check out....
Gamma Ray, Nightwish, Primal Fear, Dream Evil, Dionysus, Tad Morose, Kamelot, Brainstorm, Seven Witches, and Symphony X.