JISO

If you're on the hunt for a free and open source Isometric game with cross-platform accessibility, then JISO is the perfect one for you. It offers several resemblances with the infamous Final Fantasy Tactics game, but with the uniqueness of having been crafted for the sake of fun, rather than profit.

Furthermore, the Sun Public License governs the distribution and usage of JISO, ensuring that you as the user, have complete and legal access to the software. Being a cross-platform game, it allows you to play it on whichever device is at your disposal, regardless of whether it is a Windows, Mac, or Linux operating system.

Moreover, with JISO being freely available, it presents an opportunity for enthusiasts to contribute to the development of the game. The best part about this game is that it welcomes all kinds of tweaks and changes, while still managing to retain its originality.

All in all, if you're looking for an engaging, cross-platform game that's open source, then JISO should be on the top of your list. Its Final Fantasy similarities, creator's intentions of designing it for fun, and open-source status makes it worth checking out.