Perhaps the most obvious pattern is with Space Marines, partly because they're more developed than other groups. The big theme is storm imagery. Storm Bolters, Hurricane Bolters, Land Speeder Typhoon/Storm/Tornado/Tempest, Whirlwinds, Thunder Hammers and Storm Shields, Stormravens and Thunderhawks, Lightning Claws, Cyclone Missile Launchers and so on. Heck, even *bolter* could be seen as a lightning/thunder reference. Plus a number of chapters have some reference in their name, like the Storm Giants, Thunder Barons, and the new Storm Wardens. If you're making something, especially a weapon, for the Space Marines it would be hard to go wrong with a storm-related name - if you can find one that hasn't already been used!
While Marine flying vehicles like Land Speeders and flying transports follow this pattern, other vehicles don't. The Land Raider Crusader could have easily been the Land Raider Hurricane, but they went with a different pattern, likely due to its origins with the Black Templars. The follow-up, the Redeemer, follows the Crusader imagery, though the (introduced earlier, IIRC) Forge World variants like Ares or Helios don't, instead following a pattern of Greco-Roman god names. The Terminus pattern falls into this latter category. At a guess, I think the Crusader series is more concerned with delivering and supporting troops, while the Helios, etc. seem more focused on their own weaponry, sometimes even at the expense of carrying capacity altogether. When deciding which naming convention to follow with a custom Land Raider that might be something to take into account. As a side note, the Vindicator also fits in well with the Crusader/Redeemer, so perhaps that angle could be taken with any close-in support tank for Marines.
Rhino/Razorback could be two different patterns, just animals that start with R, or stubborn-type animals that charge. In either event if you were looking for another Rhino-chassis transport variant, something like Ram could work either way, or something like Bull could be appropriate under the second pattern.
Predators come in two standard types, the Destructor and the Annihilator, providing an easy pattern to follow, making things like Predator Ravager or Predator Eliminator fit right in. Thanks to the Space Wolves for following the precedent set by the Destructor instead of using their homeworld like the Blood Angels did for the Baal, otherwise we might have to talk about Fenris-pattern Predators. I guess having the main Imperial Guard tank named after their primarch was enough for them.
Speaking of the Imperial Guard, there are a couple good conventions there as well. One is the mythic monster pattern for Chimera-chassis vehicles, with followups like Hydra, Basilisk, Manticore and so on. The other is the pain/damage/defeat theme for the various Leman Russ variants - Punisher, Eradicator, Exterminator, Conqueror and so on, basically anything involving victory over one's enemies.
Chaos doesn't have a lot of variants, or just carries over Imperial names for many things, but you can see some patterns with words like Dread, Hell, Doom and so on. Any kind of doom&gloom, despair and corruption type word should work for a Chaos vehicle. Of course for those Khornate or Nurgle creations, toss in blood, skull, or plague as appropriate.
The Eldar skimmers missed some opportunities. Some have a nice snake pattern going - Wave Serpent, Vyper, Venom, Cobra - but then it gets thrown off with the Falcon, Fire Prism, and so on. Maybe it makes more sense in the original
Tau have that nice fish theme going, with Hammerhead, Manta, Piranha, Devilfish and so on. Kind of ironic, since these things are going to be crewed by Fire or Air caste members instead of Water caste.
Most Necron names are pretty basic, but they do have that monument theme going with Monolith, Pylon, Obelisk and the general death/life theme with things like Tomb and Immortals. Custom units named something like Cenotaph or Undying should fit right in.
Other major patterns that you folks see?