The dungeon crawler I have the most experience playing is the old Digital Devil Story: Megami Tensei on the NES. It uses grid-based traversal with 90 degree turning, but also some Final Fantasy or Pokemon mechanics. It's a neat and fun game, but can be pretty obtuse.
In my opinion, one of the main things old dungeon crawlers are lacking is some user friendliness. So if you take concepts from some of these old games and add some good 21st century game design philosopy, you'd be on the right track to success. Enjoy!