There is two ways, first, taking a classic like Dark Sun and improving on the battle system to get closer to a ToEE/KotC combat system, say more KotC but groups of enemies attacking together from ToEE was really neat.
If you find few features to add that would really improve the experience, just do, otherwise don't.
On the other hand, complexity is a good thing if you have the resources to pull it off and if the features are well designed and work well together.
The best RPG hasn't been released yet, when you look at games like Fantasy General and UFO, which are not RPG but close to the genre (tactical combat), it's clear they're better than whatever have been released in the last20 years, first, because of production value but also because of the complexity and how well every feature is designed.
The best RPG would be a synthesis between PST and KotC with some well designed additional features (without it, if perfectly designed, it'd already be the best RPG of all times), a good trading simulation of kingdom management would work well if well designed and easy to use (remotely from the party, not adding loading screens, bug-free, well optimized).
For example, a huge open world with different pre-designed scenarios randomly triggered with kingdom management and trading routes, with multiple adventuring parties which you can all control or put some on auto-mode with some directions.
Add to this different origins, from prince of a kingdom to merchant, evil apprentice wizard wanting to make a pact with demons or planning to become a necromancer and then a lich and unleash undead hordes, Orc warrior with the options to become chief and gather multiple tribes, demon prince, ...)
Add to this handcrafted complex scenarios that can trigger in your game of not with a lot of build up, handcrafted dungeons, hidden locations you'll have to scout.
So, if you have 30M €, a great team, a good taste and a vast knowledge of PnP and computer RPG, complex is good, otherwise, simple and close to the classics is best with few if any new features.
Also, don't waste resources on cut-scenes and 3D graphics.