I want something big, but not too big (AKA that wont outgrow the tank) and something that has color or personality, isnt ugly, an is entertaining. I would love some bigger fish, as the largest FW tank I have ever had was only a 29g.
I sure understand the desire for larger fish but have only had personal experience with smaller fish until recently. Now I have 6 rainbow cichlids, herotilapia multispinosa, that are growing up to adult size in a 29. They may be the centerpiece in my 125 some day but that's not certain. Even with the larger tank, I have found that I'd rather have lots of platies, mollies, corydorus, etc. than one or two large fish.
rainbow cichlids are awesome little cichlids, a pair of them would make a great display in the proposed 40 gallon tank. Add along with them a school of Buenos Aires tetras or something like that, and it would look great.