NVidia shield tablet - a few questions I have

Can we be expecting a newer model to hit the market by then and if so what new features might it have?

I haven't heard a single so don't hold your breath waiting for a new shield.

Since I have currently got an amd card (r9 270) I'll need an upgrade to the green team. What card can you recommend for primarily gaming use with intense games like batman arkham Knight but only on a 1440x900 display so it doesn't need to be superly overpowered.

Well, arkham games always favor NVIDIA so getting a good card won't be a problem. Gtx 950 performs as as good as 270 so I wouldn't get any thing less than that.

I also have a 2.4GHz router that I heard was not good enough for streaming geforce experience games to the shield tablet. Is this true?

It depends. For what I have seen, ISP connection bandwidth seems to be more important. 25 mbs of download speed at least. An ac should improve experience but the gains will be marginal.

I would like to know if what I think is correct: nvidia's GRID service let's you stream games from nvidia's servers outside of your home network and costs money but Gamestream is free, only in your home network and streams games from your pc.

That is correct. Gamesteam doesn't necessarily limit to one location you can play from your tablet in one location and stream the pc from another assuming your connections is good enough to handle it.