Maybe like SPD's A-squad?

We have Deathryuger...Not much, but it's a start. A purely evil ranger with a dark agenda and a zord to call his own (Brainwashed into darkness, but still).

I felt they were criminally underused.

Yeah. A team that either acts on their own, or WILLINGLY joins up with the show's villain.

That was the biggest problem. They never felt like a big threat and their turn wasn't all that dramatic because they only showed up in, like what, 3 episodes?

It was a good idea though. Just not enough execution.

I'm in favor of the "recurring boss" approach. They show up fairly early on, layeth down the smack as a group, then start appearing infrequently as the heroes get their acts together and power up, either in pairs or on their own to support the MotW until they finally get taken down. There's no need for them to be sympathetic, just some level of personality and character development- be as cohesive a unit, if not moreso, than the Rangers, but undoubtedly evil and unrepentant about what they do.

Y'know, ideally, anyway.

I'm on about a team though, a whole team. Single evil Rangers are great, but I want to see a whole team whose skills act as the perfect foils for the main team. But each member of the team has their own dynamic, dreams, their own balance with the rest of their team. I'd happily watch a whole sentai about two Ranger teams fighting and exploring the characters within the helmets.

Phantom Thief （Think Kaito Kid or Arsene Lupin）

Ahem... KAMEN SENTAI DECADE!