Why did Lothor only bring only bring Zurgane back with him? Choobo was incompetent, Vexacus couldn't be trusted, and Motordrone was a machine.
What about Shimazu? They did bring back the Wolfblades, after all.
Why does Tommy's morpher have a key? Because his last set of (original) powers was Turbo, and they had keyed morphers as well.
Mesogog's unusual speech patterns could be explained by Anton having both Multiple-Personality Disorder and Asperger's Syndrome. If Anton had Asperger's Syndrome to begin with, then when he had a physical transformation, it wouldn't have necessarily had to rewire his brain. In other words, it would have been Anton inside of Mesogog's body. So why would Mesogog have acted like a different person? Multiple-personality disorder. Anton created the alter-ego of Mesogog after the trauma of seeing himself as a monster. Mesogog was his repressed hatred and paranoia, possibly from Asperger's Syndrome itself, since the disorder can be accompanied by paranoia\depression\schizophrenia. If that were the case, then Mesogog would also have Asperger's Syndrome, and his speech patterns would simply be a variance on that.
Linkara's views on Dino Thunder, especially towards the end, shed some light on some matters.
The Evil White Dino Ranger's Phlebotinum Breakdown is the result of the Morphing Grid not being able to handle two White Rangers.
Trent's refusal for help initially is his mentality that he's a superhero and that superheroes never need help.
Also, Tommy needing to learn to fight for his own life is a reflection of one of his more subtle traits in the old days - guilt over his actions while under evil influence led him to be willing to stick his neck out very far indeed for others, yet reluctant to have anyone else risk themselves for his sake.
Why did it take the raw power of the Dino Gems to defeat Mesogog? Well, Mesogog wants to bring the world back to reptilian domination, just like when dinosaurs ruled the Earth. Now, what helped kill the dinosaurs? A meteor, of which the Dino Gems are fragments of.
Problem: The Dinogems got their power from being what wiped out the dinosaurs, by your logic they should have gained power from destroying Mesagog.
All of the Dinozords are named after real dinosaurs or prehistoric beasts... except for the Dragozord (dragon). This seems weird... until you realize that, up until the past couple centuries, dinosaur fossils were often believed to be the bones of dragons!
Tommy wears long sleeves in every episode of Dino Thunder because Jason David Frank has tattoos on his arms. The reason this is Fridge Brilliance is that, if the character of Tommy Oliver PhD. has the same tattoos, he'd likely cover them to teach High School. That doesn't explain why he wears long sleeves even when not working; but hey, this was Disney
People tend to think that teachers have to have morally high ground despite being human like everyone. Maybe he was covering them so parents of children attending Reefside high school wouldn't comment even when he was off-duty. They tried to get a teacher fired for having a tatoo on her back exposed while in a swimming pool in this Not Always Learning story  . It does not matter if the story is true or not.
In the first episodes (before he gets the Black Dino Gem) Tommy wears red. What was the last color he had? Red!
Speaking of colors, notice the blanket covering Tommy in Fighting Spirit. It's green and white, and a close up on his collar shows that it's red underneath. From top to bottom, it represents the struggle he had with red barely shown and green being exposed the most. Not bad.
There's a lot of symbolism regarding the fighting of the past Ranger forms Tommy had. First off, he goes backwards from Red Zeo Ranger, White Ranger to Green Ranger. Looking at it, Red Zeo was when he officially became leader of the Power Rangers, White Ranger was his resurrected Ranger form and the Green Ranger was the form with the most regrets/struggles he had to go through. This is also backed up with the scenery. Red Zeo is in a desert with water, White Ranger has a bare forest, and Green Ranger is a beautiful forest with fallen pillars. This represents his struggle in being a ranger because as time went on, the battle of good versus evil was putting a big strain on him and the landscape was a reflection of his own mind.
The Red Zeo ranger fight taking place in a desert could connect to the time Tommy was brainwashed into leading the machine empire in a Colosseum withing a desert
A small one in the Ninja Storm team up episode - as they prepare for the big Ranger-on-Ranger showdown, Conner says 'they use special powers, we use special powers'. In two fights - Conner v Shane, Ethan v Dustin - the Dino Thunder Rangers use their powers first and in Tori v Kira they were used at very nearly the same time. Why? Because the Ninja Rangers had been needled about honour in the Cyber Cafe, and even when evil they still held themselves to a standard.
Tommy having the civilian power of invisibility while Trent has the civilian power of Camouflage can highlight the similarities in their plot lines, being corrupted into evil rangers then slowly taming their power to join their friends as heroes, since they function the same way by hiding the users from sight.
Tommy, a scientific genius? Not only does he have a PhD, but he gets it a year early. Wasn't this the same guy who could never remember anything?
You could chalk it up to a number of things. Maybe the Machine empire's brainwashing for the "King for a Day" ep made Tommy smarter, and maybe Hayley tutoring him helped him with memory problems. Maybe Tommy has hidden depths we were never shown. Maybe TJ, Carlos, Ashley and Cassie managed to get something figured out for the past two teams post-Coundown to Destruction, maybe Tommy studied extremely hard, maybe Anton bribed someone after hearing Tommy having encountered Ranger business before, which would contribute to the Tyrannodrones (Putty Patroller experience) and the Bio Zords. Plus, bad memory dosen't equal dumb and it had been something of a Compressed Vice in the first place anyway.
Arguably, this could be Fridge Brilliance as it shows his growth as a leader. His forgetfulness was usually manifested by being late to Ranger battles due to the Sentai footage of the original Green and White Rangers. It's reduced a bit later as the White Ranger since they had to use original footage for all but the Zord battles. Once he became the Red Ranger, he was usually on time.
As the Red Turbo Ranger Tommy's lingering forgetfulness and lateness (he missed his own graduation, afterall) was usually explained with Tommy spending countless hours tinkering with automobiles and the Turbo Zords, trying to gain a better insight into his Turbo Powers. So, when properly motivated, even as a teenager, Tommy was always able to focus and overcome his short attention span: the discovery of the Dino Gems could have spurred an interest in Paleontology able to drive Tommy away from every single distraction in life, and devoting himself to his PhD.
Tommy actually didn't miss his graduation during Turbo. Due to spending time at the race track due to his interest in cars, Tommy opted to get his diploma earlier. And all his interest about cars is somehow gone during Dino Thunder, focusing on dinosaurs instead.
After Turbo, a man used to getting by with less sleep than most, and having been on alert for years in battles that literally involved life as we know it, if at all, continuing found himself with only grades and papers as adversaries. Even without any brain enhancements, he had been exposed to tech that would blow most of his teachers' minds, and would have gained from that. In short, he had the energy, the drive, the need to be involved in a project, and no fear of anything we would encounter in our normal lives. At some point, he also had no girlfriend anymore, whatever the future holds. Could a man like that get to a PhD at ludicrous speed? I say Yes!