Ok so I had an idea as to how this could work, it might get a tad wordy and hard to follow, and I apologise.
This takes a few assumptions and liberties, based on the fact we don't know -everything- yet.
But, let's say during/after the final battle with Bladerun, the 'Miracle Gosei Armour' is destroyed. (all it needs is some battle-damaged suit work, broken visor, etc) This ties-up/removes the plot hole of "but they already have a power-up form" for Super-Megaforce.
The Silver Ranger shows up at the start of the new series, on the run from the Zangyack. He's a [something related to KO-35/Andros, having heard stories of the past rangers i.e. Forever Red].
He lands on earth, with the Zangyack in full tow behind him. This leads to the events of 199Hero Battle. (For plot's sake, Silver can be the 'Tech Genius' of the group, and created the [Space Pirate Powers], and uses them much like Basco uses 6th ranger keys. Eliminating the hole that Gosei and Gokai fought together in 199hero)
Once the Rangers use their powers/Channel the Morphing Grid/etc, and defeat the main fleet of Zangyack forces, There's Demorphing all round (and we get our first set of Cameos).
Silver (As the Tech Genius) grants the Megaforce Rangers the [Space Pirate Power-up Mode].
In the base, Silver and [Mentor/cameo of the week] work on the [Power ranger bible] and starts constructing sets of statues/models of previous rangers, inventing the [ranger key system].
The System works on drawing the power of the previous Ranger teams for a limited time. This can be visualised in much the same way as the Suits from RPM were Materialised. (Statues disappear/teleport out of their place on the wall, appearing as Ranger Keys.)
This way, instead of "Learning the ways of the true sentai warriors", it can be a case of "We've finished tapping into the Galaxy powers, Get Ready for Transport" or something similar. This way it can stay as week-to-week content, but not going "well if they have ALL the powers, then why not do X" Also allows for silver to come up with the things he can do, without the whole "LOL IS A DREAM AND NOW HE IS MAGIC" **** that happened in Gokaiger.
Keeping it somewhat fast paced, they can then just glide through content with somewhat of the same formula, Old ranger wants to help/shows up/whatever, Gosei footage of fighting the Zangyack, Power of the week gets harnessed, Rangers Power up, Rangers change, Fight happens, Megazord Fight happens, Rangers win, everyone is cool.
Pacing it so that we don't get 400 episodes with Fuuraimaru as the megazord powerup definitely helps here as well, since a lot of those episodes were on teams that weren't in the Power rangers timeline.
I hope this kind of makes sense to people, haha. Especially regarding the statues/keys idea. Since all that needs is some simple VFX work and it's good to go. The rest is just helping explain that ****.