Oooooh! We love these questions and spending other people's money!
First of all, you got a great starter package - great choices!
For the saw, you might think about a longer rail or at least an extra rail and the joiners to combine two. A must for breaking down sheet goods. The blade that comes with the saw is darned good, you may never need a different one, but its always good to have a spare for when the other one is getting sharpened.
For the MFT, you will love QWAS dogs. Festool doesn't make them, but Bob Marino sells them.
For the vac, maybe a cleaning kit? The boom arm and the workcenter are pricey, but everytime I use them (which is several times a day) I have to grin because they are so handy!
There's a bazillion accessories for the OF1400. Get a couple of extra stops for the rail, for stopped cuts. If you are using patterns or making jigs to work with the 1400, get some copy rings (or the porter cable compatible ring if you already have PC template guides). Of course, there's router bits, but I recommend buying as you need them. Once you digest all this stuff, look at Brice's video on the MFS. If you use the OF1400 on the guide rail, you will need a strategy for keeping it level (e.g. something to make up for the thickness of the rail). The outrigger that comes with it is supposed to do that, but its a piece of junk. Search the FOG for a thread that discussed this - there are several alternatives.
Whew, I'm out of breath and you are probably out of money by now.