I was making a character who had speed as an important part of the character. I remembered one of the disciplines the old mystic class had was celerity and I kind of want the effects of it. I don't really want to multiclass into mystic as it would neither fit the character and I don't want to make them too strong because mystics get a lot at first level along with celerity.

I kind of want to know opinion on ideas of what I could replace on a wood elf rogue 4/artificer 2, or any other ideas.

(Small note, I wrote this at 3am and had no idea which forum to send this in.)

I'm not super familiar with what the celerity ability used to do, so it might be worthwhile summarising what you actually want? And is it homebrew you're actually looking for, or just recommendations on how to build the character? If the latter then Tips & Tactics might make more sense.

What I will say, is that Rogues actually get a bit of extra speed already thanks to Cunning Action (can Dash, Disengage or Hide as a bonus action). If what you really want is a full additional action then your quickest option is to take two levels in Fighter for Action Surge (once per short rest), however I wouldn't normally recommend a third class on most builds. Alternatively if you're expecting a long campaign you could hold out for 9th-level in Artificer to take haste, but that's a long time to wait.

Alternatively I'd discuss with your DM the possibility of getting a tailored magic item to help you represent celerity as you level up; for example, boots of speed or a custom item that lets you cast haste once a day or similar? As long as it's part of the same magic item progression as everyone else then it should be balanced.

Celerity

Immortal Discipline

You channel psionic power into your body, honing your reflexes and agility to an incredible degree. The world seems to slow down while you continue to move as normal.
Psychic Focus. While focused on this discipline, your walking speed increases by 10 feet.
Rapid Step (1–7 psi). As a bonus action, you increase your walking speed by 10 feet per psi point spent until the end of the current turn. If you have a climbing or swimming speed, this increase applies to that speed as well.
Agile Defense (2 psi). As a bonus action, you take the Dodge action.
Blur of Motion (2 psi). As an action, you cause yourself to be invisible during any of your movement during the current turn.
Surge of Speed (2 psi). As a bonus action, you gain two benefits until the end of the current turn: you don't provoke opportunity attacks, and you have a climbing speed equal to your walking speed.
Surge of Action (5 psi). As a bonus action, you can Dash or make one weapon attack.

That sounds a heck of a lot like Cunning Action to me. If it's Dodge or Attack in particular you want, I think allowing Cunning Action to do that prof. bonus times a day would be reasonable.

Magic Items are also a good way to add to your character without messing with class/race. scimitar of speed gives you a bonus action attack with no strings attached, although it's very rare. Which gives you a good idea of how strong that ability is.