I think if I were doing it again (in a 10 - but I may go the same way in a 20), I would find 5 or six really interesting pieces of driftwood and so place them as to chop up the swimming space into a half dozen or so cells. I'd plant the back and sides of the tank with fluffy stems (Cabomba, Parrot Feather, Rotala, Limnophilia), and put some rosettes (crypts, small swords) in the middle. That ought to break up the sightlines really well, produce the sort of blackwater bettas really like, and create a bunch of hiding places.
The other option is to go with the hygrophilia Jungle. Get some Wisteria and some Hygro polysperma and let them go wild. Not a pretty tank, but very effective.