HoMM 6 is in development, but the studio developing it is Black Hole Entertainment, the company that did Warhammer Mark of Chaos, a pretty mediocre RTS, so I'm not super enthused about that.
If you haven't, I'd heartily recommend you check out King's Bounty, and the expansion/sequel Armored Princess, if you enjoy HoMM style games, it's a very good one.