I am leaning more towards an Amazon biotope, maybe some kind of frog. I am having trouble finding frogs though, they all seem to be tree frogs.
Tree Frogs are a great way to go ... I know you some how want to stear away from them ... but a 40 breeder is a good size if it were turned on end and constructed that way. The idea of a palu is for it to be part water and part land and this could work out.
Check out this giant build with a large water area with dart frogs: http://www.plantedtank.net/forums/vivarium-terrarium/97228-225-gallon-office-setup.html
Its way cool and it show the set up and build.
Best wishes,
Wes