I think someone has done this before with Bitcoin and miners found ways to manipulate the outcome. You should find the most transparent method possible to do this, because trust is everything in Lottery games like this. I still have my doubts on both Millionairemakers on Reddit and also the Lottery system, built into Freebitco.in.
Provide a chat area for people who are participating to validate that you are playing against actual people and not some automated bots that are pre-configured to enter these games on behalf of the owner to cheat the system. ^hmmmmm^
Yes in the past we have seen a lot of lotteries with cryptocurrencies that failed, scammed or didn't work. That's why we are trying to create something new, that will be 100% trasparent on what it does and it will work around the community, not around the money!
Of course we will try our best to eliminate any sort of manipulation flaws. We will work hard to maintain the highest level of security.
As Edgew said, we might think to introduce a general chat room, but for sure we are working to find a way to prove that every user in a room is a REAL one and not a bot or ourself.