Whats a good small/medium sized indoor pet with a good life expectancy?

Hi, I'm quite the pet loving person and soon I wanted to get another pet but I'm not sure on what to chose. I already have a gerbil, fish and dog etc but I wanted to get something else to go in my bedroom that's quite small or medium sized with a life expectancy with more than just something like 2 or 3 years, So what would you recommend me getting? Also I'm 15 but don't let that shorten your options I am quite experienced with animals and look after mine constantly.