You know, now that I think of it, I don't think the current model of isekai wouldn't actually be a good base for a game, even though nearly all of them use "game mechanics."
I mean, isn't one of the big draws of the isekai genre, the fact that they're using "game mechanics" like status screens and leveling up, in a non-game world?
An isekai game would be just a basic fantasy game.
Although, if you use the literal term of isekai , which is "another world", then there would be a couple of games that qualify.
The Final Fantasy Tactics Advance games are a major example. So would Final Fantasy X. Star Ocean 2 as well. And if you count travelling through time as "another world" then games that have that would count as well.
Think it could be a fun beginning for a game. You start off in your home town doing normal things, get summoned to a fantasy world and go from there. Maybe have it on subsequent playthroughs that you can get different home world items to use in the new world. ---