GaoGaiGar/WMG

G-Stones are crystalized Spiral Energy
They're the same shade of green, they react to emotion (courage is just fighting spirit by a different name), they have limitless potential. Not much of a stretch here. Similarly, J-Jewels are also crystalized spiral energy tinted red by the Red Planet's manufacturing processes. Guy's Evoluder form is a human body that has become capable of consciously harnessing spiral power--a prequel to humans as we see them in TTGL--hence the "holding GaoFighGar together by force of will" scene in FINAL.
 * This would make the Loud G-Stone the precursor to the beastmen Anti-Spirals, would it not? Because, a backwards spiral... is still a spiral. (YEAAAAAAAAAAAHHHH)
 * Could also explain the Genesic Machines in FINAL: The remaining energies of the G-Crystal not absorbed into Ghaleon being formed into new mecha. (This troper personally finds the notion that 5 shards of the G-Crystal just happened to both contain the Genesic Machines AND avoid destruction at the hands of the Sol Masters, plus they were apparently still in energy-form when they shot off to block the Sol Wave.)
 * Alternately, G-Stone radiate Spiral Energy.

GaoGaiGar humans will eventually become the Anti-Spirals
...because man will grow proud of their G-Stone/Spiral Power-based technology, never realizing the threat of the Spiral Nemesis until long after the legend of GGG has faded from memory.

G-Stones are made of Thiotimoline.
The determination-activated spacetime-warping properties of the G-Stones are consistent with those described in Dr. Isaac Asimov's The Micropsychiatric Properties of Thiotimoline.

Everyone alive at the Bittersweet Ending of FINAL survives.
There is one more Gadget Tool mentioned in the Genesic GaoGaiGar Eyecatch, called the Galeoria Road. From what we know of the Galeoria Comet, this would probably create a massive ES Window through which everyone could escape the collapsing universe. So Everybody Lives, And the Adventure Continues... with the GGG crew Walking The Multiverse. Further evidence: The very last frames of FINAL are a shot of the thought-destroyed Galeoria Comet. Make of that what you will.
 * This... this actually makes a huge amount of sense. Even setting aside the Galeoria Road, they also had other space-warping tools available like the Bolting Driver and access to ZA POWAH, so there's no reason GGG shouldn't have been able to go home... except that they were still exiled from the solar system by the UN. There's no home left for them to go back to (except for Mamoru and Kaidou, who aren't officially part of GGG) so the obvious solution is to go find a new one. The reason they fed Mamoru and Kaidou that story about being trapped is because the kids are the only members of the team who still have friends and family waiting for them on Earth and they didn't want to force them to choose between their civilian friends and their fellow Braves; the adults must have made this decision for them while they were both unconscious after the Final Battle.

Kouryu and Anryu both suck at landing.
I've had this idea in my head ever since the scene where they both crash in Final Episode 7. True, it was because they were falling from space, but what if that scene was telling us something else. Consider that Kouryu has trouble stopping in space, and Anryu does the Traditional Ryu sibling faceplant in Super Robot Wars. It would be funnier cause it breaks the usual pattern.

Hana was being targeted, either covertly or actively, by the Zonderians.
It may be more than just unfortunate coincidence that she was in pretty much every location that Pasder and the Machine Kings attacked, even when it was outside of G-Island City. They knew that Mamoru's power to Purify their creations was a huge threat to them, so getting rid of him was a high priority. If they were aware of their Toy Ship status, she'd be perfect bait for drawing him in for them to eliminate. (This troper can only speculate how much of a disaster it would have been if they'd ever made her into a Zonder...)

The GGG crew returns to earth 8 years after FINAL ends
Given how we learned during the final episodes the comet itself passes Earth every eight years, they just have to hold out until the cycle starts again. Then when they return, they have to save Earth from yet another threat.
 * That would first and foremost require that they could survive for 8 years. And that's without a viable foodsource. Remember, Repli-Earth was destroyed when the Pas-Q Machine was destroyed.
 * They survived on COURAGE!!!

Galeon is the reincarnated form of Mufasa
Memetic Badass Papa Wolf King of Beasts. After overseeing that his son Simba reclaimed The Pride Lands, Mufasa allowed himself to enter the cycle of reincarnation and was later resurrected as an even more Badass Humongous Mecha, going by the alias of Galeon. He watches over Mamoru because he reminds him somewhat of his own son, and has allied with humanity For Great Justice!