Had the idea for making a playable Drider race for my players, but rather than making a whole new race it might be fun to have it piggyback off of the existing Centaur as a variant since both are basically creatures with non-humanoid bottom halves. Let me know what you think

A player who wishes to play as a drider can do so by using the Centaur race with the following changes:

• Your drow ancestry overpowers the spider-like instincts of your transformation. Your creature type is Monstrosity  Humanoid instead of Fey, and you gain the Fey Ancestry and Sunlight Sensitivity traits. 
• Your Hooves are replaced with a Bite, which act as unarmed strikes that deal piercing damage equal to 1d4+your Strength or Dexterity score (your choice). Your bite can also be used in place of hooves for your Charge trait. 
• You lose the Equine Build trait and instead gain the following:

Arachnid Build: You have a climb speed equal to your walking speed. In addition, at 3rd level you can move up, down, and across vertical surfaces and upside down along ceilings, while leaving your hands free. At 5th level, you ignore movement restrictions caused by webbing. 

I have actually thought about this, but never made the connection between the Drider and Centaur for the traits. 

Seems like it would be really fun to play.

Feels like a straight upgrade.

• Monstrosity is probably better than Fey because you're not only immune to spells that target humanoids, but you're also not on the "evil and good" list of creatures.
• Adding Fey Ancestry is purely an extra feature.
• Arachnid Build takes the weakness of the centaur and makes it a strength. Yeah you lose carrying capacity but how many games care about that? Even in games that track encumbrance you just give your stuff to someone else to carry.

For the sake of balance, I'd probably remove Charge/Bite completely. As far as I know, Driders have roughly the same face as a regular drow so I don't see why they would have a bite attack and removing those features would counterbalance the other upgrades you've given them.

Quote from scatterbraind>>

Feels like a straight upgrade.

• Monstrosity is probably better than Fey because you're not only immune to spells that target humanoids, but you're also not on the "evil and good" list of creatures.

I think I will change this to be a humanoid, but keep the Fey Ancestry aspect so that the player race Drider is more of a Drow and less of a spider monstrosity. I was on the fence about this before, but I think you're right here. 

• Adding Fey Ancestry is purely an extra feature.

Correct, and one that the Drider has. Even if not a Fey themselves, it makes sense for a Drider to have Fey Ancestry based off of the elven ties. 

• Arachnid Build takes the weakness of the centaur and makes it a strength. Yeah you lose carrying capacity but how many games care about that? Even in games that track encumbrance you just give your stuff to someone else to carry.

True, but it makes sense thematically for a spider-like creature to not be incumbered by climbing. Its ability to climb is in line (mechanically speaking) with the Spider Climb ability of the Dhampir lineage. The only added bonus is the ability to ignore webbing after 5th level, which also arguably will come up very rarely unless you build into it or have a spider-focused campaign. 

For the sake of balance, I'd probably remove Charge/Bite completely. As far as I know, Driders have roughly the same face as a regular drow so I don't see why they would have a bite attack and removing those features would counterbalance the other upgrades you've given them.

 Driders have access to a Bite attack. Also, if I remove the natural weapon and Charge ability, this no longer feels like a variation on the Centaur and would just become its own racial option, which is fine, but my original goal was to design it as a variant not its own thing. Considering the drider has 8 extra legs, I see no issue with it keeping the Charge feature. 

The existence of the drider bite has always bothered me. I'm not sure if they bite with their drow mouth, or if they have some kind of bonus spider mouth, but either way, I'm unsettled.

On a serious note, I absolutely love this idea, but I'm inclined to agree with scatter. Removing the Charge/Bite entirely would probably be a good idea in terms of balance. If I'm honest, I'd be apprehensive about a player having Monstrosity as a creature type, too (personally, I'd want to make them Humanoid; maybe give them back the extra carrying capacity as a consolation prize) but I don't think it's the end of the world if you leave it as-is.

I would leave it as a Monstrosity because it is one (just like the UA Thri-kreen), but it would need a balancing factor like sunlight sensitivity. I know, stuff like that is disappearing from 5e for player options like races, but IMO that’s what your Drider race needs.

Quote from IamSposta >>

I would leave it as a Monstrosity because it is one (just like the UA Thri-kreen), but it would need a balancing factor like sunlight sensitivity. I know, stuff like that is disappearing from 5e for player options like races, but IMO that’s what your Drider race needs.

 I think changing the creature type to a Humanoid could be an easy way to help balance it. Monstrous Driders are monstrosities, yes, but in the same vein monstrous Centaurs are also monstrosities rather than fey, so I think the door is open for having that differ for the player race. One could maybe justify it from a flavor/lore perspective by simply saying that player race Driders are a subset where the Drow portion manages to not give into the more spider-like instincts that tend to come with the transformation. 

I have been playing around with giving them Sunlight Sensitivity as you suggested, and that might be a decent debuff to help balance out the other aspects without removing any of them

Heeeey, couldn't driders be used as mounts? (for a campaign that has multiple people that could use it)