Waiting in the Summer/WMG

Yamano Remon is Morino Ichigo.
They have the same voice actor and the same Evil Laugh. Their names translate to Lemon of the Mountain and Strawberry of the Forest, respectively. Remon wears a school uniform that is out of place with the rest of the school, yet matches the uniform for the school from Please Teacher / Onegai Twins. The two also share the same hair color, eye color and Deadpan Snarker traits.
 * Their identical Evil Laugh with the exact same voice is a dead-giveaway.
 * Likely she still has her Soap Opera Disease, which was something along the lines of a kind of cryogenic stasis that freezes the aging process. Likely Ichigo goes through this every time she goes to sleep, effectively freezing herself for eight hours a night, meaning that she ages at two thirds the rate (about) of a normal person. Take into account that it has been ten years between Please Teacher and Ano Natsu de Matteru that means she would only physically be about six or seven years older. Morino looked more like a gradeschooler than a highschooler and Yamano looks more like a middleschooler than a High School Senior.
 * Episode 2 shows the cast of High School 1st years drinking a strange brew and acting inebriated. Were this really Ichigo she could have bought it easily with her normal ID since she would be about thirty two years old here.
 * Guarantee that at some point she is going to say that she is only 17 when someone asks her how old she is.
 * Alluded to in episode 8, when Manami is puzzled by the fact that Remon is still in high school, and she responds, saying she "is an eternal 17-year-old".
 * My pet theory is that Morino is now a Professor of Sociology/Anthropology, and this is a long term field research expedition. her age would explain the hard lemonade, and grant money would explain how she can afford to $1k+ telephoto lens.
 * She has a driver's license and a transforming robot van.
 * She's Did anyone see that coming?

Mio is a naturist. (CONFIRMED)
Mio has been shown reacting very oddly to the idea of people looking at her -- wearing sweaters and the like on top of bathing suits in lieu of letting others see her in them, for example. In addition, every shot we have seen of her home life shows her casually nude, indeed with no clothing nor clothing storage (dressers, hamper, etc) in sight. The answer is clear -- Mio (or her family) is a naturist, and she doesn't want others to know due to nudism's scarcity in Japan, and the view that most people in Japan would have to a young naturist girl.
 * Mio is seen in multiple episodes being absolutely horrified at the possibility of people seeing her as a Ms. Fanservice. She even wears a hoodie to the pool and beach instead of letting people see her in a swimsuit.
 * Multiple times throughout the series we're shown shots of Mio at her house. In each and every instance, she is nude, with no clothing or clothing related furniture in sight.
 * This becomes confirmed in Episode 7, when Mio is accused of being an Exhibitionist by a stalker. She confesses that she's not exhibitionistic at all, instead her family are nudists and she's always been raised that way. Tetsuro does not react with shock or disgust, instead telling her that she has to trust her friends with her worries and fears like that.

Ichika is related to Mizuho from Please Teacher.
She has a similar hair color, and in episode 5 mentions having an older and younger sister. She may have come to Earth because of Mizuho telling her how wonderful it is, and hence Ichika's motivation to find a that place she mentions to Kaito several times.

The place Ichika is looking for is actually the place where her parents, Mizuho and Kei, met for the first time.
Lake and trees. Which place with a lake and trees hold a special significance in the Onegai franchise? The lake where Mizuho's ship landed and the trees at the lakeside are where she teleported to the ground, where Kei was awakening from a standstill.
 * Actually, that place is so special in the franchise that no other series in the franchise has shown a scene there.

Someone will not survive to the end of the series (JOSSED)
The framing device is that the narrator (presumed to be Kaito) is recollecting his experiences during "that Summer." However, all of the narration appears to be setting up some form of tragedy -- Kanna, for example, does not seem to be handling the love triangle very well. In addition, we know that Kaito received grievous, possibly fatal, wounds from the crash landing that Ichika arrived in. In addition, Kaito's narration when discussing Kanna's was more introspective, suggesting he was remembering someone no longer around.

I propose one of two results will occur before the end of the series: Kaito's body will reject the healing nanotech that is keeping him alive, resulting in his death, or Kanna will hit the Despair Event Horizon and attempt (or succeed) at suicide. Alternately, Kaito will have to be taken to receive medical attention outside of Earth, or Kanna will need similar help that Kaito received.
 * With Kanna recovering from her depression in episode 10 (including confessing to Kaito and being confessed to by Tetsuro), the next obvious candidate is Mio -- who is acting near stalker-levels of creepy now. Consider that in almost every scene Tetsuro has had by himself, Mio has either been shown just around the corner listening in, or shows up after everyone else leaves to interact with him. The only real way she could do this is if she was stalking him throughout the day. At the same time, she's coming across as emotionally distant and detached -- or perhaps just mature. Either she is handling the situation much better than the others, or she's one bad day away from snapping like a frail twig.
 * Played with in the end: Due to a series of events the series ends on a Downer Ending, with  There's a Darkest Hour moment suggesting some hope, namely , but the series ends on a very sad note.   The cast have no real lingering problems at the end (although Mio certainly plays up the stalker angle the last few episodes, and Kanna is still openly heartbroken), however.