A Greek world would be amazing! I suggested one as part of an archeology career refresh - find statuary of the Greek Gods and monsters as well as displayable weaponry - and such a world would easily accommodate beach lots, I think.
The trouble with *just* a beach world is a sense of been there, done that. I know that the theme hasn't been properly fleshed out yet - what in the Sims has? - but I'm afraid if the concentrated on more beach stuff it would be at the expense of some other great things they could be working on. It's a double-edged sword.
That said, I'd kill for beach volleyball, pool/water bars, kids being able to play games in the water like marco polo, a boardwalk with carnival games and prizes and a carousel, surfing, and improved mermaids with underwater lots to explore.