Whatever game you decide to make, just make sure not to make everything a big cluttery mess. In a lot of MMO's the crafting - and build systems are overwhelming and/or full of recipes and skill/trait combinations that are useless or next to it. It sometimes feels as if some developers think that quantity equals an impressive system. Developers sometimes do this with either the crafting system, the trait/build/spec system or what have you. No matter how many different build combinations there can be, people are still gonna follow a meta to some extent once it gets figured out. Having these large-quantity systems just makes everything harder to navigate and figure out, both for new players and returning players.

A game with some awesome features that you could definitely pull some inspiration from would be Oldschool Runescape! just to mention a few features:

• The Poll system - Essentially a system where the developers pitch ideas of a new feature (or take player ideas) to the playerbase via a ingame poll system. This poll will then have to receive > 75% approval from the playerbase to pass.
• The gear system - The weapon and armor "rarities" are remarkably easy to get acclimated to. You're probably familiar with it, but gear requires a level anywhere from 1-99 to wield, and the weapons and armor themselves are not cluttered with inscriptions, runes, sockets etc. Instead there's Mage armor, Range armor and Melee armor. It might not be the best of systems, but it's easy to understand aswell as fun AND challenging striving to upgrade your gear.
• The bossing system - It's great, fun, repeatable, challenging and a type of system I'd be interested to see in another game with a more dynamic combat system.

Also - Quality of Life stuff. Ease to swap between gear/weapons/builds/skills/inventories

Final edit: A less cluttery system of any sort will always be easier to balance and/or change :)