It's pretty obvious that Loki is suffering from the severe power creep that went on an on over the years. However, I don't think a total rework is required. Just a few touch ups here and there. Loki's design is meant to be simple yet synergistic by nature, and that simplicity must be preserved.
Decoy should lose its health bar for sure, so it can actual act like a potent decoy at any level of play. This should be enough to make it interesting, especially if it generates high threat.
Invisibility doesn't need a touch up : invisibility powers are broken in WF and always were. As of today, it's still very powerful.
Switch Teleport is the only power that should get a rework. However, the base principles should be conserved. You want Loki's third power to still be a mobility power while also being a power you can use to displace enemies. This is why I came up with the following idea :
New 3rd ability - Shortcut - 25 energy :
Tap the ability button to teleport at reticle location, with a maximum range of 10/15/20/25m. Enemies around the teleportation point in a 3/4/5/6m radius will suffer a knockdown.
Hold the ability button to teleport enemies at reticle location within a 6m radius to you, with a maximum casting range of 10/15/20/25m. These enemies will be knocked down after the teleportation.
Radial Disarm is as great and powerful as it ever was. Disarming enemies in such a big radius pretty much 4th power worthy. However, it's useless against the infested, so we can simply add another effect : Radial Disarm permanently reduces the speed of infested units by 40%.
This effect would NOT be affected by power strength, so Loki can focus on his usual two main ways of building : range and duration.
If it's still not enough compared to other "ultimate" powers, we can decide to simply lower the cost of radial disarm from 100 to 75 base energy, thus giving to Loki the unique property of having lower energy costs than most warframes, allowing you to focus less on power efficiency in your builds.
That's about it.
Loki doesn't need much to become an interesting pick again really.