Now that it will be getting cold soon I was thinking that maybe with my next fish I should choose ones that like cooler temperatures. My house doesn't get cold but it's not warm enough for tropical species. I do have my tropical tank heated but I was thinking that if there's plenty of fish that like cooler water than maybe that would make more sense?

I have fantails and snails in my coldwater tank

White cloud minnows? Red rosy minnows? Dojo loaches? Hillstream loaches?

Really though unless you love some of the above fish and your tank will stay within the right temperature range (and remain at stable temperatures - you don't want it frequently fluctuating) than it would just be easier to get a heater. They aren't a lot of money and won't add much to the electric bill provided you don't have loads of aquariums and this way you'll have more choices since most fish will need some heat during the cold months. Unless your house is really well insulated maybe(?).