>>723616923 (OP)
Gunna type up a lot of ideas:
Distinction between front row and backrow for attacking, with weapons having different ranges. Ranged weapons have ammo. Maybe a leveling system like Dragon Slayer II: Xanadu where characters get more proficient with pieces of equipment as they use them, or get more out of consumable items the more they use them? You could make having to use stuff correctly or make use of certain features of a consumable a requirement to get more mastery out of it.
Having a big world to explore like an early Dragon Quest is a must, but maybe make dungeon crawling first person. Mapping by hand is cool but I'm also totally fine with auto-map. Going the Etrian Odyssey route and doing the mapping by hand in game would be coolest, with the caveat that maybe you need a Dumapic like spell to see your exact location and cardinal orientation.
You could do either blank slate characters you assign different classes to like DQ3 or Wizardry, or go the DQ6/7 route and have named characters you can do vocational changes on. If you go the latter route, party chat is huge and greatly appreciated to flesh them out more.
Setting wise do whatever you'd like, but I'd love to see a cool blend of medieval and sci-fi fantasy. Maybe do overall mostly medieval with hints of sci-fi here and there.
A cool idea for dungeons is to have roaming NPCs in them like Wizardry V, but you could have them show up on your map like FOEs in EO with something like a life detect spell. I wouldn't include Wizardry V's swimming unless you did more with it as it felt really barebones and was really undercooked.