In Star Wars there's a lot of starships that either have no canon design or no canon name. That's common for several ships from the post-Vong war era. Ships like the Nargi-class Pursuit Frigate, Ranger Gunship, Galactic Battle Carrier and the Blue Diver. There HAVE been fan made designs for some of these ships, but never canon.
The five ships I'll be listing here are either ships with a Canon name and no official design or somewhat canon ships, but with Fan names and history. Spending so much time on Wookieepedia is mostly how I found a lot of these.
The Legacy-class Star Destroyer
The ship or at least it's design was made by Howard Day (Although this model was made by Codeuser). While I'm not 100% sure, I wonder if the Legacy was based on this ship below and here Starwars.fandom.com
The Centax-class Heavy Frigate
A canon starship, but a fan design, or is it fanon? Ships with a similar design can be seen alongside an Acclamator on Anaxes during an Imperial parade. The model itself was made by Evillejedi. Maybe this is where he got the idea from. (You'll have to scroll a little to find the image in the link)
The Hammer-class Picket Ship
Another ship model made by Evillejedi, found in the Republic at War mod. Well most of the ship models pictured here come from that mod except for the Legacy. The name is Fanon, but it's based on a ship called the Separatist boarding craft. Starwars.fandom.com
The Technounion Frigate
Sometimes I think I've seen this ship referred to as a destroyer. Again model was made by Evillejedi, turns out it was based on a space yacht. A head canon I have for this ship is that it was built by the Techno Union, usually built for important members of the Union or gifted to someone who had done something for the Techno Union, and on occasion sold to wealthy people who could afford them. Because they were already heavily armored vessels, many were converted into warships by the Techno Union to be used by the CIS.
Starwars.fandom.com's_space_yacht
Patrol Frigate
Another made by Evillejedi, sadly the image for this ship can no longer be found on Wookieepedia. It was removed after what I've started calling the "Great Purge". While some articles I could kinda understand, this one and another ship called the Separatist Star Destroyer were deleted from Wookieepedia. The image I took in the EaW model viewer is a recreation of the image. I do remember a few years ago finding the image on some Spanish Star Wars wiki, but I can't even find that anymore!
all of these ships are better than anything disney has made