I've used R18, DMM and several other JAV hard copy and digital content provides.
Here are some suggestions.
1. Provide better quality video files.
Most of the available content is poorly encoded.
I possess a lot of media files transcoded from my personal DVD and Bluray collection, for digital storage.
So I know quality of R18 videos could be improved significantly with proper transcoding/encoding practices.
Although I strongly dislike DRM content, I would suffer through it if the video quality was great.
That's not the case at the moment.
2. Identical or similar access to content between R18 and DMM.
Most of the times, I end up using DMM rather than R18, because R18's offerings are not as complete as DMM.
It would be great if R18 also had access to all of DMM's contents.
Seeing as both sites seem to run off the same servers, it seems feasible.
3. Exclusive Content.
May it be VR content or Subtitles, it would be nice to see exclusive content once in awhile that is only available on R18.
In its current state, R18 only differs from its competitors by the fact that it has an English website.
Most users can still navigate your Japanese competitors' website with google translate.
Set yourself apart.
4. Community oriented Tag (Genre) and Review system.
Many people use separate 3rd party websites to organize, find, and classify their Japanese AV content.
This is because the system on R18 and DMM is not very thorough or comprehensive.
A lot of the times, there are not enough Tags, Genres, or indicators for titles.
Most of the multi-actress "amateur" content do not have proper actress tags, even though they might be a famous actress.
Providing and option for the subscribed or premium members to add tags, genres, description and reviews would allow R18 to become a true one-stop source for all Japanese AV related information and content.
A tag or description system can be mostly driven and moderated by the community via upvote & downvote system.
ie. A subscriber to that Channel or Purchaser of the video is able to add a new genre/tag to the video.
If it's a newly submitted genre/tag, it's added to the description and starts with 2 upvotes.
If two other members who have subscribed to the channel or purchased the video in question, find that genre/tag to be misleading or incorrect, they can downvote.
The 2 downvotes cancel out the upvotes, and the genre/tag is removed from the description.
Same can be applied to things like Actress names and Reviews.
Would be surprised if any of the suggestions posted on this thread gets implemented