Umihara Kawase is a fantastic series. I enjoy it and have gotten plenty out of it. However, it's never sold well. Why? They're perfectly well-designed games with an original mechanic that's not often imitated. If it were up to me to design one and make it more popular, what would I do without making it unrecognizable?
1. Enhance content
Content, more than gameplay, is what sells games and initially catches people's attention. Hence why E3 is filled with insubstantial cinematic trailers. Swinging in Umihara is intuitive, fun, and empowering. But where are we doing it? Non-descript water areas with spartan looking gamey blocks. How about some variety? There are plenty of interesting places that contain water. How about rainforests, hidden temples, luxury pool resorts, and a giant fish tank? Sayonara even had levels that took place inside a giant bathtub, with suds, sprinklers, and soap! How's that for interesting? Sure beats the deluge of lakes, streams, and oceans we've had throughout the series. How about some variety in the obstacles and enemies as well? I have evangelized the games to many people and given them a chance to play it, and they quickly stop playing after the first Game Over. You know why? Because all the levels look mostly alike. There's no motivation to persevere to discover where the next level will take you. The general public doesn't see much of a difference between Taki and Hatoba fields for example. More level and enemy variety would fix this.
Oh, and make the levels a bit bigger. The levels in Umihara games have always been miniscule, with the Original's levels being like 9 screens worth of space tops.
2. Accessibility Features
Sayonara was the installment that boosted the popularity of the series significantly due to its accessibility features. The first major thing it did was put individual levels first, and unlock Survival Mode later. In any new installments, have individual levels and Survival be unlocked from the start. During Survival modes, start with... I dunno, 5 lives. Similar to Kanzenban, include continues and make whatever field you're on the final field if they take too long (say, the time limit of 30 minutes). So far, this sounds like an exclusive imitation of only the more recent games, so here's where I'd make a diversion: eliminate text-based tutorials. The tutorials from Sayonara were terrible. They constantly interrupted the player and required reading in what used to be a wordless series anyone could play without having to know any language. In fact, when I first played the 3DS version when it came out, my initial feeling was "Oh no, they've lost it". Bring back the short, sweet, skippable tutorial cutscenes from earlier games. These are all design decisions that would make the game more accessible, without sacrificing any amount of challenge.
3. Marketing
The marketing for Umihara games isn't terrible. It's functional and gets the message across. However, it only preaches to the small existing fanbase and isn't likely to hold anyone's interest. You know what would? Commercials about Umihara and her swinging abilities that have nothing to do with the game, sort of like the average 90s commercial. You could have ads of Umi wall-climing a tall building, hanging sky-high from a support beam where a bunch of men are eating lunch, or getting out of a dangerous/adventurous situation. It would make the gameplay look cool and get the message of its practicality across without actually having to show people what the game looks like, and they'd be able to imagine it while playing the game. If anything, that would be an improvement over the current marketing.
4. Audio/Visuals
The music would have to sound good and not too hokey. Repetitive chiptunes aren't going to cut it anymore. More variety would be good too. Two of my favorite songs in the series, Danchi and Pout, have a somewhat different musical style than most of its other songs. As for visuals, there is official fanart of Umi wearing a kimono, a dark blue swimsuit, and even a bikini. Have those costumes be playable as well, and make it so you only have to beat a Survival path with one of them to unlock her Profile Picture. Don't change the music based on which costume you're wearing, because players might like the costume, but not like the music.
So, those are my suggestions in creating a market success for Umihara. What do you think? Are these good suggestions, or do I need to be banished from this website for thinking such things? Think long and hard about it before you answer.