You have 7 month until release, but there's also the potential of franchise fatigue, additionally when factoring in TTGC and FC, so I'd say if you want to play XC3, just do it. I assume "not finished" means you have at least started, so you own the games? Then just play them after. Perhaps XC3 would even make you want to revisit the other games?
I don't feel like going through the whole thread, so sorry if this was discussed, but what's the likelihood of Mio and High Entia girl being descendents, perhaps daughters, of Melia and Nia? Mio basically looks like Gormotti Nia, down to the Core Crystal. High Entia girl's brown hair could also indicate that she's the child of a mixed High Entia and a Homs. Dunban? lol Though on the other hand only mixed High Entia are left anyway, so her mother doesn't necessarily have to be Melia.
There's the whole black vs white theme going on and so far all the flutes also seem to come in those colours, with Noah and Mio potentially swapping theirs? Does that mean the actual threat could be gray? Or could gray be the protag group, as a sign of two sides having joined?