Well, my knowledge is more with power metal bands, so I'll see what I can drag up with my preference
- while it's not the only ballad they have done, it's the only duet. Interesting bit of connections here - the female vocalist in the song, Candice Night, is married to Ritchie Blackmore, who had a band called Rainbow, which Dio was a part of before joining Black Sabbath. She also is part of the band Blackmore's Night, which plays a nice set of relaxing folk rock/new age.
- hate to bring up two song examples, but this one almost escaped my mind. Helloween has a history of being (almost) the godfather of power metal. They also set the tone for the creativity found by a lot of the bands in the genre, and this foray into reggae isn't any different.Gary Moore - Thunder Rising - the electric blues artist has played a variety of music, although his primary style has definitely been the blues. The album this song is from, Wild Frontier, is different from much of his catalog. His stuff was also generally different from his time as a part of Thin Lizzy.
- Blind Guardian released a whole album of rearranged songs mixed in with several covers, including Mr. Sandman (Pat Ballard), The Wizard (Uriah Heep), Spread Your Wings (Queen), and Barbara Ann (Fred Fassert). Otherwise, they haven't really done covers that I've seen (there may be some lying around in some singles) - they have been known to play Barbara Ann live, which is fun to witness as a change of pace.
- this is a man singing, former frontman Andre Matos. This is a cover of the Kate Bush song Wuthering Heights.
- while opera has a huge influence on metal, this is the only cover of an opera song I know of. Manowar is better known for their ridiculous album covers and being the loudest band, but the band has good variety.
- Nightwish went full jazz on this original song off of their most recent studio album. They've shown some good variety throughout their career, but this was a first. The band has gotten even more creative since they've shed their operatic metal approach - probably helps when you've achieved commercial success on their level.Probably could dig up more stuff, but this got me to finally start exploring more music again...its been too long for me.