A soft reboot is that they don't completely disregard the events of a last movie but take a new view of it with style, casting, tone and other things. Since Hasbro was revealed to take over Power Rangers from Saban, fans have wanted Hasbro to continue the film series. Some people want a sequel and some people want a reboot. I would just like a sequel, but for it to a big success, it would have to handle some problems from the first one. Hasbro property G.I.Joe had a soft reboot with the sequel concentrating more on a smaller cast, less weapons and working on problems from the first.
I think the tone was fine, some people think it was too dour. What some people who aren't diehard fans now but were fans as kids want it to be campier or more 'fun.' I would like more humor but a smart humor, a humor more like Marvel cinema series than campy movies like Batman and Robin or something all over the place like Green Lantern. Also, I say keep the cast, don't recast. But build on their personalities and backstories like Zack and Trini.
In Suit Battles
The biggest complaint of the movie was there wasn't enough time of the rangers in the suits. I agree. Now that the Rangers can morph, they can mix it up, have battle scenes evenly spread throughout the movie. The biggest element of Power Rangers is the mix of teenagers' lives and their ranger duties.
Less Technical, more Opaque Zords
Now that Hasbro is in charge maybe the zords can look less like Transformers and different, maybe more opaque and less technical. X-Men 2 did different styles changing the interior of the X-Wing and the hairstyles of the girls. Batman Forever completely changed the Batman suit, Batmobile, Gotham city's designs but kept the event of the last one. They can change the zords' look and ranger suits. I never really liked the ranger suits, maybe less silver and more of their regular color and less complexity.
Green Ranger is the biggest draw to a sequel, with Tommy and the Dragonzord. I personally think the Dragon zord has to be huge, has to be the biggest draw. I think the producers and designers have to look at Pacific Rim and Gundam and Halo for big bulky mech. I would have Dragonzord to be bulky and large. Also Tommy would have to conflict and be convert with all the Rangers. Have a personal stake in the character. Have the Rangers get to know Tommy first and then have evil Tommy affect them personally. Tommy had the unique ability to affect the Rangers in their personal lives and Ranger life.
Soft Reboot examples
Batman Forever - kept last movie history, changed suit, vehicles and sets. Also recast lead.
G.I. Joe Retaliation - dumped last cast, reduced characters, changed tone.
Superman Lives - recast, style same, continued from first two movies, ignored others.
So a Power Rangers sequel/soft reboot should change some styles for zords and suits, focus more on suit battles, incorporate Tommy into both personal and Ranger battles, soften tone, and keep cast.