I would only call The Old Gods necessary. The rest are only useful if you play as the specific culture they address. For instance, I have all the DLCs but I've yet to start as a Muslim or Byzantine character and I can honestly say I haven't even noticed the difference. I'd say Sons of Abraham is near-necessary as well but the broken holy order system just
ed up my current game for a few decades and I'm still pissed off.
RoI is the same. Unless you are planning on playing as a Raja in India then don't bother. Wait for a few weeks/months till it's on sale, you'll still get the map expansion.