I'm assuming level cap for Beta will be 20 as all the Legendary gear in Alpha was level 20. Just an assumption and I'm assuming it'll be between 20 - 50 for retail (as most MMO's usually cap at 50 (some rare ones don't)).
Character slot wise I'm going to guess 3 slots, one for each character class as you really can't specialize your characters to do one thing in this game like you can in most MMO's (I mean you can swap abilities in the tree at will). Plus honestly there really is no reason to go beyond 3 slots, anymore and thats just ridiculous (who needs more then one Titan, Hunter, or Warlock at any given time).
I'm going out on a limb here by saying I doubt there will be no more new classes until the 3rd or 4th DLC. I'm assuming the first two will add more Strikes and probably more planets as it seems like only the Moon and Mars will be in retail besides Earth (this is just speculation, I'd honestly be really surprised if Venus and Mercury were added into retail). Most likely they'll also contain new multiplayer maps/modes and probably new armor sets and weapons. Who knows what Bungie and Activision have planned for this game, I mean this is the only game Bungie is developing for them atm and they plan on keeping it going for a long while by the sounds of it.
Interesting app, I'll have to check it out when it's released.