ditto and kudos on giving them more space. I feel bad enough for having two in a ten gallon aquarium and sometimes think I'm no better than those people at the office that used to keep beta's in a cup of water.
I've been pondering your question too for a few months.... I've purchased a 36"x18"x18" exo terra enclosure and plan to build it out for just my two bombina's . As far as plants, I think any that work well in a terrarium will be good. You can google "terrarium plants" and find all sorts of opinions and suggestions. Same thing on the water side of your tank, just google "aquarium plants".
Just be sure that the plants are not heat loving as your fb's are not.