No offense to Zoroark Guy, but all types have varied, strong Pokemon. Type advantages don't really matter when it comes to making a mono team. For example, most people look at Bug as not varied, weak, and only a few type advantages (Psychic, Dark, and Grass).
This is completely incorrect. I have made a mono team of every type, and they all work quite well.
Things you should look for:
1. A type you are fairly comfortable with.
2. Dual Types. (There are some exceptions to this, such as Weezing for a tank in poison.)
3. Pokemon you would actually use in an OU team. (ie: Ferrothorn for grass.)
4. Think about common strategies for those types, such as weather-based teams.
Finally, the only restrictions for a mono team are whatever restrictions are already in place, like OU, UU, RU, NU, and PU. (Of course, all of your Pokemon have to be of the same type.)
Let me know how that works out.