Question about my special metal stick?

[Trickster]

How much could I conceivably do to one scimitar as a player? Is an +3 Adamantine Scimitar the farthest I can go?

[Skrum]

If I'm understanding this correctly....

There is no base system for crafting or creating magic items. As a player you have no ability to make or get any particular magic item (aside from the specific class features of the artificer class), unless the DM makes that kind of thing available to you as part of their own game.

That said - in terms of what items can do, they can get way more interesting than adamantine +3

A wakened Dragon's Wrath weapon (can be any weapon type) is a very rare weapon with a +2 enhancement bonus and deals an additional 2d6 damage with every strike of a damage type matching a particular dragon. 1/day it can unleash a cone of the same energy in a 30 ft cone that deals 8d6 damage and DC 16 Dex save to reduce it by half.

That's just one example. There's a ton of magic items.

[Unoriginal]

How much could I conceivably do to one scimitar as a player? Is an +3 Adamantine Scimitar the farthest I can go?

Are you talking about finding, or about crafting?

[Mastikator]

As far as a DM says. Anywhere between nothing to rod of resurrection and beyond.

[Trickster]

I was referring to crafting systems, but in the event that three isn't this question is basically invalid lol but thanks for responses nonetheless!

[Unoriginal]

I was referring to crafting systems, but in the event that three isn't this question is basically invalid lol but thanks for responses nonetheless!

There is a magic item crafting system (technically, several).

What's important need to keep in mind is that a) you generally can't add magic past the first crafting b) unless your DM does things differently, you need to find the formula to craft the specific magic item, and a formula is generally one rank of rarity higher than the item itself.

An Artificier has different ways to craft magic items, but they're generally not long-term.

Other than that, well, only your DM can answer.

As for how powerful you can go, well, that depends what kind of power you want. A +3 weapon is pretty great, but many don't find +X options as interesting as, say, a Scimitar of Speed, which is a +2 weapon allowing to do one attack as a bonus action.

Which can be very good if you like using your main action to do something else than the Attack action.

[Amnestic]

There's no 'hard' magic item crafting system. There's two 'soft' ones - one in the DMG, one in Xanathar's - which are mostly mutually exclusive and also don't have much in the way of details, leaving it mostly up to the DM.

Can you make a +3 Vorpal Flametongue Scimitar of Speed that also rubs your head and calls you a good boy every time you kill an enemy?

Nothing really says no, nothing really says yes either.

[KorvinStarmast]

There are a number of artifacts that are +3 blades which have other features as well.
Take your +3 blade, make it adamantine, and then for each beneficial feature you add beyond that also add a non beneficial feature.
And there's your artifact.

[Skrum]

Nothing really says no, nothing really says yes either.

Perfect summation of the crafting rules. Also the skill rules. Also the vision rules.

[Theodoxus]

Perfect summation of the crafting rules. Also the skill rules. Also the vision rules.

100%. D&D, the most words written of any rules lite TTRPG where the final arbiter is 'ask your DM' or 'Rulings not rules' or 'Everything is made up and the points don't matter.' Take you pick.

Kinda makes me wonder why we have discussion boards and forums and whatnot.

[JonBeowulf]

Can you make a +3 Vorpal Flametongue Scimitar of Speed that also rubs your head and calls you a good boy every time you kill an enemy?

I want one.

[OracleofWuffing]

I want one.

For legal reasons, we are obligated to advise you that nine out of ten weaponsmiths strongly advise you to not allow a vorpal sword close access to one's own head.

Now, uh, sign here, here, here, aaand initial here.

[kingcheesepants]

As others have pointed out crafting is a very up to the DM type of deal. But that being said there are lots of generic variants that can be applied to scimitars (how many can be applied at once is again very much up to the DM).
But here's the list of applicable variants as close as I know it
+1, 2, 3
Adamantine
Bloodshed Blade
Crystal Blade
Dancing Sword
Defender
Dragon Slayer
Dragon's Wrath
Drow Weapon
Flametongue
Fool's Blade
Forcebreaker Weapon
Frost Brand
Giant Slayer
Hellfire Weapon
Holy Avenger
Luck Blade
Mind Blade
Moon touched
Nine lives stealer
Sword of Life Stealing
Sword of Sharpness
Sword of the Planes
Sword of Wounding
Sword of Vengeance
Vicious Weapon
Vorpal Sword
Weapon of Throne's Command
Weapon of Warning
And of course Scimitar of Speed as a special version of a scimitar specifically.
I probably missed a couple but that covers what I know.