Some of the fastest cars in GTA Online lack HSW upgrades but are still at the top of their class. Unfortunately, this trait isn't very valuable in next-gen consoles since Hao's Special Works can make vehicles significantly faster by comparison. The difference between vehicles with this performance upgrade and those without it can be in the 20-30 mph range.
Rockstar Games has given old vehicles some love in the past. It wouldn't be too farfetched to assume some of the following cars could potentially be looked at for a future HSW upgrade.
Note: This article is subjective and reflects the opinions of the writer.
The 811 (Super) and other fast cars that should get HSW upgrades in GTA Online
1) DR1 (Open Wheel)
Open Wheel vehicles in GTA Online are based on NASCAR cars, which can go up to 200 mph in the real world. Sadly, the game's equivalent is significantly slower and less impressive to drive. The fastest of the bunch is the DR1, which only goes up to a decent 124 mph.
For reference, some HSW vehicles can go up to 150 mph. A regular vehicle modified by a random guy being significantly faster than an actual race car doesn't make any sense. Not to mention, stuff in the Open Wheel category cost nearly $3 million, and their performance still leaves much to be desired.
It would be great if at least one Open Wheel car feels blazingly fast to drive. Thus, something like the DR1 definitely deserves an HSW upgrade.
2) Z-Type (Sports Classic)
The Z-Type can currently go up to 126.25 mph, making it the fastest non-weaponized automobile without HSW upgrades in the Sports Classic category. This car has an old-school look that makes it stand out even amongst its contemporaries.
The Z-Type was around since Day 1 of GTA Online and was in its single-player counterpart. Thankfully, it doesn't cost $10 million in the multiplayer game as it does in GTA 5.
3) 811 (Super)
The 811 was faster than the Deveste Eight, but the latter got an HSW upgrade, whereas the former got nothing. Unfortunately, the 811 isn't as relevant as it used to be on next-gen consoles because it's no longer the fastest in the Super class.
For reference, the 811 can go up to 132.5 mph. Its lap time was nothing special by comparison, as it was near the middle of the pack. Making it even quicker would be a nice touch that fans would appreciate.
4) Pariah (Sports)
There was once a time when the Pariah was the fastest car eligible for regular races in GTA Online. HSW upgrades shattered that record, so it would be interesting for this automobile to get the upgrade. On a similar note, the Itali RSX would be another good option, but it's merely an honorable mention here to focus on other vehicle classes.
The Ocelot Pariah can currently go up to 136 mph, which is still impressive in today's metagame. It's too iconic of a vehicle to simply fall to the wayside in the face of a power creep, especially since it's not the fastest anymore.
5) Weevil Custom (Muscle)
Benny's Vehicles aren't associated with HSW upgrades, but that doesn't mean a future car shouldn't get one. The Weevil Custom is the fastest non-HSW option in the muscle class. It can go up to 137.50 mph, which is very good.
GTA Online players can already get the regular Weevil for free. Ergo, it would be a nice progression if they could do that, pay for conversion at Benny's, and then get Hao's Special Works to install some performance upgrades.
At the very least, it would be different from the usual ways players progress in this game.