Saltwater is great. If you have good tap water to start... it is all about the salinity.
But, just be prepared to spend a lot more for everything.
I have no desire to ever go back to saltwater again simply because of the cost.
^ This. A standard 40g setup will run $800-1000 easily if you buy new or fairly new equipment, plus the salinity costs.
You can do a mini reef eg in a 20 long for way less money, especially if you 1) get good live rock, 2) stock lightly, 3) go slow, 4) do small but consistent water changes (10%/week or every two weeks), and 5) use some tricks to help you out, e.g. a chaeto fuge (where macro algae grows under a bright light and water circulates through slowly; the algae growth helps strip out nitrates).
My 15g mini reef is just a basic 24x12x12 tank ($10 used), dual T5 light fixture ($40 used), aqueon pro 200W heater ($25 at Petco sale), Aqueon 500 circulation pump (free with a previous tank), 30lbs live rock ($2/lb from other hobbyists), 15lbs crushed coral ($10 at Petco), HOB air-driven breeder box w/ LED light and chaeto (used as fuge - swapped with Blair for it), and then my corals/fish ($100-150 total). So around $250. Here's two pics (regular light and actinic light).
I also agree with Ogre44 on the planted tank idea. I really like smaller SA cichlids and/or oddballs because I can do plants without them getting ripped up, and since a lot of my environment is drab tan walls (I work IT) or my basement at home, I like the splash of green in my planted tank(s). I'm not doing CO2 or anything crazy, just the T8 fixture I built at the MCA workshop with a single 'plant and aquarium' Aqueon brand bulb.
Like this one:
Maybe a sw reef tank or sw aggressor tank. If you already have some big tanks try looking into some triggers or some other aggressive salties. It might be a new challenge to keep things interesting.
Some fun ideas for an aggressor tank that is FW could include something like a tiger datnoid. Some of the bigger options would require a far larger tank but a datnoid could do fine in a 40 long/55 gallon for a good while then a 75-125 as an adult. Or you can go with smaller 'predators' like an African leaf fish (55g is plenty) or even the smaller African butterfly fish (20 long is more than enough).
Honestly you could do a mix of all these things, figure out what you like, then upgrade to a bigger tank for that one idea down the road. For example I have a three tier rack in my basement that has axolotls on the bottom, a pair of CA cichlids in the middle, and my reef tank on the top...then my 55g oddball/catfish tank nearby has a Senegal bichir, a datnoid growing out, African leaf fish, and assorted catfish. I still haven't decided which of them I like the best so I have a mix.
