Microsoft really ought to buy Unity. Selfishly, it might prod them along to expand their .Net Core (or whatever they are calling it this week) offerings further.

XNA used to sit in this space, and it was a pretty nifty little game development platform for C#. Monogame has carried on, and expanded the target platforms nicely, but hasn't really made many other improvements, from what I've seen, getting back into it after a few years away.