Game partners could be angry if I use all credits to prevent timing out for almost a year
I would offer to make 2 types of credits: usual credits and special credits. Usual are which are sent by players to their own credit-accounts on the site, using payment systems, or which are got as transfer from other players. Usual credits can be used to buy premium features, to bet in games on credits, or to convert in cash and take out of the site (less commission of payment system). Each won game on credits brings some comission for site developement. For example, FFA 4 players game with bet 10 credits from each player gives total 40 credits for winner and may be 0.4 to 2.0 credits as a comission. Winner can convert his 38.0 to 39.6 credits into cash using the exchange rate, published on the site, and can take this cash out from site, if he wish, using a payment system.
But special credits, got as a prise, can't be converted into cash, they though can be used to bet in games as well as usual credits, or they can be used to buy premium features.
Seeing of credits ladder would be useful for to understand who bring more money to site development, who can play on credits etc.
What about new premium features, I would offer something about player's image on the site. May be a picture near the name on ladder and on personal page. To show not only country but and city/town/village where player living.
May be also to save history of chat with friends.
As a funny variant is to make auction who will make first move in FFA games, that affect to NAP. This is advantage, of course, but it may be neutralised by other players who don't wish to have NAP.
Analog of this is in poker, where player can make "straddle" (twice big blind) to make his move after all other players.