I am merely speculating with the connotations of the titles from different viewpoints...
Orchid: A flower
Morningrise: A time of the day
My Arms, Your Hearse: Parts of the body/lifeless object
Still Life: An artform
Blackwater Park: A place
1) we need an animal.
Orchid: Linked with death
Morningrise: Linked with the beginning of something new
My Arms, Your Hearse: Death and life intertwined
Still Life: Static life --> Death?
Blackwater Park: dark, pestilential place
2) maybe something concretely in movement?
Orchid: A Black Sabbath song
Morningrise: Well, no actual artist comes to mind. Luckily three out of five is enough to make some sort of hypothesis...
My Arms, Your Hearse: see above
Still Life: A Fates Warning song AND a 1970's prog band
Blackwater Park: A 1970's prog band
3) Something to be linked with the band's musical favorites.
4) Each title has had a different vibe. The next one should be no exception, even if certain themes have proven to reoccur every time...
The result:
1) "Turn loose the swans"...
2) Or rather "Dead swans floating": The swan being dead, it is a continuation with the theme that flows through their albums. The sawn is the animal that is needed, and since it is floating, there is the motion. Then what about the musical connotation? Well, we all know Mikael's fascination with the cover of Black Sabbath's first album, I suppose. On the inside of the gatefold LP we have an inverted cross and a text describing the cover, wherein is a line that says "the dead swan floats upturned" (not entirely correct, I do not have the CD here and my memory fails me). So we have the connection with an artist that has influenced Opeth.
On the other hand, the band may not be this analytical. I think I need some sleep...