In my opinion if we’re talking adult content wise Resort Boin is one of the games I would most recommend you license and translate. At the time of release it was an absolute favorite on message boards, probably due to how beautiful happo bijin’s art is and the fact that overall it’s pretty clean by eroge standards.
Other probably smaller but traditional yet beutiful eroge would include ai no chikara and ai no katachi, as they’re older now they might be easier to acquire.
It’s a shame illusion won’t work with you though because there’s got to be more international fans than there are in Japan and here everyone is just stealing their content for free when they could profit greatly from working with you.
I think licensing Starless was a good move. I think anything from sei shoujo is a safe bet and making the changes you intend to are probably going to widen the appeal (although I understand from the backlash from the hardcore fans I think ultimately it will be a good decision).
I would say the best way to make an eroge as a learning tool (may not be ideal to implement) would be to have the game in Japanese with the option to put the mouse over the cursor of a kanji and see the furigana of said kanji, allowing the learner to look it up on a tablet or other dictionary like tool. Then secondly you should have an option on the page to just change the text on that page to english to show the entire translation for those who give up on difficult sentences.
