Hey lightlord, I have played a few times on your site, never won anything though, but it was fun. Let's get started with my suggestions for your gambling site.
1. Layout & Design
Okay, so when I look at bitvest.io, I see fun games, but the layout is on the poor side. For example the background of the playing game is too monotone. The menu shouldn't be at the bottom of the page, but at the top, so it is easier to navigate through your site. Also I would recommend to change colors of the menu.
2. Rakeback and house edge
A casino or gambling site with a low house edge attracts more players. When people get a % rakeback from their bets, they are happy they didn't loose all their btc and therefor will keep playing at your site. For example level 1 users gets a 1% rakeback, level 2 users gets a 2% rakeback, and so on untill max level, let's say level 20. So a 20% rakeback would attract so many more players. This is just an example, u can do these rakeback rates based on what u think is fair.
3. Accepting other coins
Also a good thing to do is adding more coins to your site, this will increase the amount of public u are targeting. For example directbet.eu has been accepting more coins over the time, now they have users from all kind of different cryptocurrencys. For example Waves, it is a good coin, but not on shapeshift(yet).
4. Adding new games
Adding new games such as poker, higher or lower and a real live btc price chart betting would be cool to see. Also this will get more traffic to your site. Or think of a new unique game that would be fun to play. Mayby like a player vs player game with a small house edge.
5. Add ddos protection
Since alot of sites are getting ddos'ed, such as poloniex and other huge sites, it can be that your site will be a target too. So cloudflare's ddos protection will prevent your site for ddos attacks. So u have a 100% uptime and this is good for the trust in the site and it guarantees users security(for example users don't want the site to go offline while they are making a bet).
6. Add a FAQ
On your site is only a help page, but it misses a FAQ page. Some users that are new to bitcoin gambling will not know how to play or deposit or whatever, therefor a FAQ is really useful to help new players.
7. Quick Support
For every site, I think it is important to have 24/7 support. Or at least 12 hours a day.
Since problems can occur when betting/depositing/withdrawing and a user always wants to get his funds asap.
8. Contact/social media page
This is really needed, since social media will have a good influence on the amount of players your site will have. If u don't have a twitter account for bitvest.io, I recommend to register one, and run a twitter campaign for a couple of weeks to build up your account, and update your account daily and make some tweets everyday. I just took twitter for example, same could be done woth facebook or any other social media platform.
Will be editing this post, for more suggestions.
There is already many coins that are accepted (afaik).
I know that DDoS protection is apparently free from cloudflare and coinbase use them for ddos protection also!
Regarding, adding more games, poker and blackjack seem good choices for adding them (though this may make them seem to much like 777coin - technically their sister site as they are owned by the same owner))
Giveaways are stated many times on this thread (to be more frequent) and social media pages on twitter and facebook are also good choices for this and to deliver information to customers and can act as a fast way of getting support?