The first thing I'll say is thanks for wanting to research before running out and buying the first monitor you see.

What you want to take into consideration are the following factors:

Space - No matter the species, monitors are space hungry. They require large enclosures (relative to their size) with a deep substrate for digging/burrowing and height for climbing (even the non-arboreal species will climb). Filling the enclosure with as much usable items as you can is a great idea. Cover the sides and back with a climbable surface (cork sheet or bark, fake rock walls, etc.), and place multiple climbing branches/rocks around the enclosure. They also require many spaces for refuge; each optimally will have its own temperature range.

Food - With the species you currently keep, it would be a good idea to add a monitor that will utilize the food items you are already using. Invertebrates (from your lizards) and mice/small rats (from your snakes) are perfect food items for varanids that reach a small to medium size.

CB vs WC - This is a no brainer for me. I personally will never purchase another WC animal again, and with the abundance of quality, CB monitors available I would recommend picking a species which has been produced in captivity. The available species increase every year.

Budget - Monitors are not cheap. Even the smaller species will use a lot of hydro, take up a lot of space, and eat like no tomorrow. Make sure you can afford to invest all that is required to properly maintain one of these magnificent animals.

Species - This is a personal selection. I have recommended small to medium size for you based upon the food you already have, but actual species selection is something you should do on your own. What type of animal interests you? Slender, arboreal types? Aquatic types which require lots of water? Giant behemoths that will eat in two years what a pair of ball pythons would eat in fourteen years? The choice is really up to you, but for the sake of the lizard, pick a species that you can care for properly.

Chris