The most ethical would be to reach the original dev before proceeding to resume. There is a chance he would agree and, possibly, make your life easier to continue the work. If it happens, great!
But... if it doesn't happen, oh well... it's up to you to say "well, the dev hates it, but why bother?" and embrace any possible consequences of going against his words. Needless to say, it will certainly be unethical, as you're explicitly going against the dev words.
And actually, the consequences will probably be "nothing", since well, just like
Umbra_2079 just said, we're in a piracy site, so most of us are already rulebreakers, right? Maybe most of us aren't the ones who leaks, but the majority of people here do not hesitate to happily click on that download button after an update arrives for a game we love (which most of us doesn't care if it was sent by the dev or not).
Well, maybe something could happen if you try to monetize this work with Patreon or other things, but even in this cases, there is a chance that nothing will happen at all.
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Apart from the ethical side... speaking as a player, my concern would be about the game
quality. Whenever a developer creates a game, in most cases, he creates a lot of content. Games usually needs a story, characters with personality, good dialogs, scenes and so on. When it comes to the artistic side, we're talking about modelling or drawing characters that will be compatible to their personalities, as well as secondary elements that ideally will perfectly match the plot and eroge scenes. There is also music, sound effects, game play, etc etc etc
All of these elements comes from devs seeking to bring to reality what is on their minds, so, it is safe to say that every single game in this board carries a developer style. And people who were captivated by a game enjoyed that style. Style is a personal thing, and it typically involves the dev's own personality.
That's why, for example, most people who develops femdom games probably enjoy femdom themselves, since it is much easier to create something that you like than trying to give life to something that you're really not into. It won't be natural in most cases.
It can be done, but it typically requires much more effort by the dev.
So... do you see? It's not simple to take a work from another person, put your own work into it and not affect the essence of that game. Sure, I'd love to see abandoned games losing that hated yellow tag (or at least getting a second thread), but I (and I guess most people as well) would
not like to play a poor continuation for that game, especially when it doesn't fit well with what the original dev was creating.
A good result is far from impossible to achieve, but it surely requires some effort. But if it does, well done, a lot of people will appreciate that (including me).