Could maybe an angel fish work? They tend to grow more tall than long, so a tall hex tank would be nice I don't know if they qualify as "big frikken fish" though, but they grow to be pretty large.
i'd not cram two angels in there (not enough territory), but one might be ok.
a mated pair of lorteti puffers would work if several thousand hiding spots were provided. (carinotetradon lortetis are just as aggressive as bettas but have "killer" teeth to go with those short tempers). lortet puffers can essentially be kept with NOTHING but other lortets of the opposite gender. and even mated pairs will go after each other sometimes. but the upside is that they are STUNNING fish. absolutely amazing @ 2" each.
the paradise fish is a close relative of the betta that grows to about 4". while temperaments vary, most are okay with peaceful fish like livebearers and can hold their own against mildly-aggressive fish like dwarf cichlids. paradise fish can and will kill other anabatoids of similar shape or size. males cannot be mixed in a single tank but can be kept with a harem of 3+ females. but 30g isn't big enough for anything but one male paradise fish and a school of small tetras/cories/livebearers. some huge benefits of the paradise fish are that it is extremely hardy, tolerant of a wide range of water conditions, and enjoys a temperature range of 50 to 80 degrees Fahrenheit. this is a fish that in the Victorian era was regularly shipped to Europe from Thailand sealed into milk-cans. you'd almost have to kill it with a brick.
Why wouldnt a thirty gallon be enough room for two angels. I have a thirty gallon and I keep two angels in it. And I have fry where they have spawned. I just now removed the fry. And the angels are getting ready to spawn again. I dont think they are cramed in at all. The rule is a inceh of fish per gallon of water. According to my caculations that is a perfect fit with plenty of room. I will post a picture later. I dont have any ammonia problems or anything.
If you do get a paradise fish, be very very careful what you put it with. Mine is only about 2" (hasn't grown any bigger, I've had him for almost 2 years) but he is a nasty little bugger, will rip out the other fishes eyes if it gets a chance (its kept by itself).
An angelfish can certainly grow big enough and is stately enough to make a huge presence in a vertical tank. And if you want him livelier just drop in a feeder fish!