Yo - just a random thought for you fine folks here since you're probably aching (like me) to play more Undertale and the wait for it is excruciating:
If you've never played To The Moon or its sequel, do it!
The best parts of Undertale and Deltarune for me are the mysterious themes, the open-ended speculation, the incredible music, and an amazingly moving story. To The Moon and its sequel has all of them.
It doesn't have any alternate endings or a "battle system" or anything - it's just a point and click adventure game - but it'll probably help you fill the gap as you wait for more Undertale goodness to come out.
It's available on PC, iOS, and Android, and all of the versions are great so you can't go wrong any way you choose to play it.