SwitchingQuylthulg: <Penguincw> Taking those one by one:
<I have some more questions. Firstly, for a draw no-bet, if the game is drawn, do you refund Lines 1 and 2, or do you delete the whole bet? Is there a difference?>
I refunded both lines. I'm not sure if it's even possible to delete the bet at that point.
<Also, how do you proceed with being Bookie for a leg? Do you have all easy bets, all difficult bets, all interesting bets, some combination, what?>
Rule 3: <there is no such thing as an easy bet>.
There are bets with very likely winners, but they aren't in any way <easy> bets; the players aren't trying to predict the outcome, they're trying to beat the odds. A bet where the odds are easy to estimate could be described as "easy", but the odds are never as easy to estimate as you'd think; there's always some wrinkle. (Your Coin Flip bet might be an exception to this.)
By that definition, a "difficult" bet is not one where the winner is completely unpredictable, but one where the odds are very hard to estimate accurately. This kind of bet is fun (not least for the Bookie), but it has to be done the right way; if people realize they don't have a clue about the odds, they're simply not going to bet, or will only place very small bets.
Accordingly, a good "difficult" bet is one where the odds are incredibly difficult to estimate accurately, but don't <look> the part - one where people think they know what will happen, even though they actually don't.
One bet of this type I particularly liked was the Clean Sweep Special from the 2014 Olympiad: will more teams have a perfect 8-0 score after 2 rounds in the open section or the women's section? People thought "open" was a sure winner because it had a big lead after round 1, but it didn't work that way at all. But bets like that should be used very sparingly; the game is supposed to be fun for all players, not a "can the Bookie outsmart everybody?" show.
A more typical and reusable example of a "difficult" bet would be any game that involves high rating differences (>100 points), or one famous player and one relatively unknown player.
In any case, <some combination of different types of bet> is almost definitely the best option. Different people like different types of bets (and many like variety), so we should give them different types of bets, both in terms of "easy"/"difficult" and "normal"/"unusual". "Normal" bets (X vs Y, Daily Double, Pick Three) should be the most common, but exotic ("unusual") bets also have their place.
Try things out and see what people like. Not every bet has to be popular - indeed, it's a bad idea to only use popular bets, as there should be something in the game for <everybody> and not just the majority. But popular bets should be used more often than unpopular bets.
I generally limited myself to 2 (rarely 3) "unusual" bets per round; I didn't use very many truly <weird> bets, and never more than one per round. The exception was Exotic Day (<wordfunph>'s birthday, April 24), with five unusual bets, all on the weirder-than-normal side.
You won't get <wordfunph>'s birthday, of course; you'll get <Annie>'s birthday on July 26. Considering the difference between her approach and <wordfunph>'s, maybe that could be No Exotics Day :-)
<This next question is another question that I'm probably going to figure on my own (but will ask anyway). How do you advertise the game? Should I be advertising (it's free).>
I think advertising is the area where you can set yourself apart from all previous Bookies and be unquestionably the best yet. I posted a ton about the Bookie game in tournament forums, which is a good way of advertising it to Chessgames users; this is almost certainly the main reason my legs had more participants than <wordfunph>'s and <Annie>'s legs. But no one has really taken Bookie advertising outside chessgames.com yet; there's a lot of untapped potential in social media marketing and advertising the Bookie game on other chess sites. If you tap that potential, and get new people to join Chessgames so they can play the Bookie game, you'll be a hero.