The Hero and Rab are basically Jack-of-All-Stats type characters who almost end up becoming Master of None while not quite getting there in practice - always outclassed by the specialists (Jade, Veronica, Serena) but still very reliable characters. However, it seems quite obvious as to why they both end up as this - they're related by blood. It runs in the family! Who's to say that if the Hero did train on Angri-La like Rab, he would know more magic.
Rab's lower physical capabilities are also easily explainable - he's old. Sure, he's healthier than a lot of older men would be, but he's still way past his prime.
It makes sense to why Eric lost to Jade in the Masked Marital Arts tournament. Eric relys on status effect setups, Damage Over Time prep spells and crowd control weapons for most of his damage, while Jade has unmatched burst damage on single targest. Of course she would clobber Eric easily when she can beat him down quickly before he can posion her or put her to sleep, and would lay him out far faster than his spells would wear her down.
Morcant looks a lot like King Carnelian. Toriyama's Only Six Faces strikes again, huh? Actually, it's foreshadowing - Carnelian was possessed by Mordegan most of the story, while Morcant is revealed to have been possessed by him during the Era of Heroes. This is more of a case of History Repeats.
In Spite of a Nail, some of the monsters you fight in the "main game" are still present in the postgame despite taking place before Mordegon's rise to power, with Calasmos taking his place. Why is this? Because they were already there before the initial Wham Episode. They're stronger in the postgame because Calasmos is empowering them - not Mordegon.
When encountered in the main game, Tatsunaga has essentially gone insane and you're doing a Mercy Kill. However, in the postgame, Tatsunaga is still able to be calmed by Miko - of course. Because you're at least a few months in the past - before he lost his mind.
When one finally gains the Sword of Light, it's obvious to longstanding series fans that it's the Sword of Kings, aka Roto's legendary sword. That is very obviously the Sword of Kings; But it is later on upgraded even further to the Supreme Sword of Light, which... Changes the design. One might think this creates a plothole, how the final iteration in a Stealth Prequel would have a differing design from the one of Dragon Quest I-III, but then one remembers that the original Sword of Kings was broken by Zoma before Erdrick/Roto had a new one forged. Evidently, the succeeding sword was fashioned after the blade's penultimate state.
A few monsters you fight early in the game mention a dark master. Most players will assume it to be Mordegon... but only a few such Dora-in-Grey actually say who it is. It might actually be that they are referring to Calasmos.
The rematch with Mordegon in the postgame is much harder than the fight against him at the end of the story, despite now being in a much weaker form. The difference between them, however, is one of tactics: having originally been a human mage, it's only logical that he would be more talented and more experienced at fighting with magic and trickery, rather than the physically-oriented brute he became upon powering up.
An easily-missed bit of brilliance in the fight against Jasper Unbound - look at the bahlib, an enemy he'll summon for backup. Purple hair, wields an axe? He's essentially made a monster equivalent of Hendrik his subordinate!
King Carnelian was possessed for 16 years, and his dialog implies that his true self was aware but repressed the entire time.
The gist of Booga's subplot is that he turns people into monsters, and all the monsters you see around Octagonia used to be human. This contains quite a bit of Fridge Horror:
Octogonia had an orphanage. Uh oh.
If you visit the orphanage during the occupation, most of the children are fine. The monsters can't get in there because of the same reason they can't go into the campsites - the goddess statue and all that
Did any of these humans-turned-monsters leave Octogonia? How many went after you?
Same with the Vikings in Sniflheim. Gyldigga essentially sent them to their deaths once they realized the heroes were coming.