**Disclaimer**: Tech requirements change and evolve over time. Arenanet (and most other software suppliers) have the right to change their system requirements for their products as they see fit at any time. Your computer build today is not guaranteed to work tomorrow.
With that out of the way, let's talk tech. Rather than provide specific part recommendations, I'll recommend a top-down queue of upgrades that could be made and the improvements you can expect to see:
1. **CPU** - GW2 tends to use a single core at any given time, but having more cores will improve general quality-of-life by allowing you to maintain more concurrent applications at once (Ex: Discord, web browsers, etc). Speed can be a bottleneck up to a certain point before shifting to a different part of your rig. General guidance is that bigger is better within a given processor generation, but comparing across different generations is not as straight-forward. Later generations generally perform better across the board compared to their previous generations, but not guaranteed. Specific chip manufacturer is not important for GW2. _When purchasing, verify that the CPU interface is compatible with the motherboard you intend to use_.
2. **GPU** - Improves the visual aesthetic of the game. Not quite as important for GW2 compared with the CPU, but still pretty important. Likely to be the most costly single-component upgrade in your rig. Recommend using the system requirements for other games/applications being used commonly in your rig as GW2 tends to have lower graphical requirements. Improvements here will let you switch to higher graphical settings in the options for GW2 and make use of some of the optional graphical checkboxes. This can be bottlenecked by the motherboard depending on the PCI express version supported by either component.
3. **Internet speed** - While not a specific part in your rig, improving this will provide a direct measurable performance boost up to a certain point. Additional bandwidth may be needed to support a larger online footprint (multiple online applications and/or users at once). Connecting via ethernet where possible as opposed to wireless can also provide noticeable gameplay improvements up to a certain point.
4. **Solid-state drive** - Having GW2 installed on a solid state drive will allow game files to be accessed quicker by your rig compared to storage on disk drives. This improvement would be for adding a solid-state drive to a system that doesn't already have one. Upgrading a solid-state drive to a newer model probably won't have a significant impact for GW2. As an aside, your operating system should also be installed on the solid-state drive to significantly improve boot times.
5. **RAM** - Adding more RAM to your system may not directly impact your GW2 gameplay, but it will accommodate additional supporting applications that players tend to have simultaneously running. Improvements to GW2 gameplay will probably not be noticeable going above 16 GB, but your mileage may vary by user. _When purchasing, verify that the RAM type (DDR4, DDR3, etc) + speed is compatible with the motherboard you intend to use_.
6. **Motherboard** - Assuming all of the above improvements are made, this will likely be your next major bottleneck. Not nearly as important to upgrade so long as it is compatible with the other components listed above.
7. **Power Supply** - No direct impact to GW2 gameplay, but something to keep in mind if improving several of the above components at once as it will likely increase your power draw for your rig.
8. **2+ Monitors** - No direct benefit for GW2 gameplay, but will likely impove quality-of-life for using support applications.
EDIT: Added comments to check compatibility between CPU+motherboard and RAM+motherboard.