As one of the eight original vanilla frames, DE has been rather indifferent to Loki. Among the originals he is the frame that have seen the least amount of change and half his ability kit suffers as a result of that. Decoy is entirely useless in higher levels and Switch Teleport is too situational outside of spy missions to see any valid use. Ironically both of these abilities have amazing augments but it should not take a augment to fix a ability. They need to be looked at and have some changes done to them. A full rework that introduces entirely new abilities is not necessary for Loki. Just some tweaks and changes.
Decoy needs some way to survive at higher levels without being destroyed instantly. At the current moment it doesn't get any additional durability from power strength nor does it inherent any bonus from whatever defensive mods that Loki has equipped. Giving it Health/Shields/Armor that is multiplied by X amount of Loki's could be a start or give it a phase where it initially absorbs incoming damage and then adds it to its total health. Decoy could also have its health entirely removed and just run on pure duration alone. It is a hologram after all.
Either way, Decoy needs some kind of change that allows it to withstand or ignore attacks in higher level missions because at the current moment it simply just not worth using.
As i said earlier, Switch Teleport is too situational outside of spy missions to see any valid use and this needs to change. Where i particularly find Switch Teleport lacking is in interaction with enemy targets. Currently it only briefly confuses a enemy target for 2 or 3 seconds which is entirely useless. In the soon-to-be 7 years old open beta trailer for Warframe we Excalibur, Mag, Rhino and Loki fighting a bunch of Grineer until Lech Kril shows up and he becomes their main target. Loki at two key moments uses Switch Teleport. First is to switch with Rhino causing a heavy shock wave which blows away all the nearby Grineer grunts. Second is when Loki switches with a random Grineer lancer to avoid getting hammered by Lech Kril's hammer, the one he switched with is random Grineer who promptly gets squished by the hammer. Neither of these things are possible to pull off even after 7 years of Warframe and i find that disappointing.
Allied targets that is switch teleported should receive some kind of buff or perhaps just create a basic shock wave upon being switched. This would also encourage cooperation with allies. Imagine if the buff gave a ability strength buff and a surrounded Loki switches with a Oberon who proceeds to reckon the S#&$ out of the enemies thanks to more power strength. Just a example though.
Enemy targets that is switched should be fully susceptible to both their own attacks and that of their comrades. Currently if Loki switch with a enemy bombard, the bombard just becomes disoriented for a few seconds and his rocket will pass through him which is really really disappointing. At the bare minimum, the period at which Switch Teleport confuses a target should at least be 10 seconds which is affected by duration. Even better would be if Switch Teleport inflicted a fully stacked radiation proc on enemy targets as well as making them susceptible to both their own and attacks from other enemies so something like switching bombards into their own rockets with them taking increased damage from it would be possible.
Those are the core issues that i find the most pressing with Loki. Invisibility and Radial Disarm does have a few issues but i think they are rather small compared to Decoy or Switch Teleport. Being Loki's main tool of survival, Invisibility is pretty short with a duration of just 12 seconds. Chroma for example has a duration of twenty-five seconds on both Elemental Ward and Vex Armor. I am not asking for that much but increasing the duration to 15-18 seconds would be a pretty good move. Being able to recast Invisibility before its duration runs out would also be much appreciated.
Loki could also use some refreshment in the ability FX/graphics or whatever its called. Many of his abilities look rather old but this is not a very glaring issue.