Personally, I hope that Crossrail might instate it's own (lower) fares, simply to instil some competition - but you're right, if the go for the Watford Junction inwards style of ticketing, there'll just be the one interavailable fare. But those trains all stop all-stations.
AIUI▸ all the recent extensions outside the traditional zones have either adopted existing LU zonal fares that match the price of existing
NR» fares, (e.g. putting the stations in zones 7, 8 or 9); or they have introduced new 'pseudo zones' as per the Watford Junction model.
Watford Junction is in a lettered 'Zone W' in
PAYG▸ internal operational terms, there are also Zones B, C, and G elsewhere. You won't find any of these on the
Oyster▸ fares map, they are all covered by the concept of 'special fares apply'.
Someone has written a wiki article about this, as you'd expect:
https://en.wikipedia.org/wiki/List_of_stations_in_London_fare_zones_7–W You can also find in the outer reaches of the map that zones are skipped, or the number isn't printed - so again using Watford Junction as an example you'll see on the Oyster map it is beyond Zone 8, but not in Zone 9, because number 9 is not printed.
I can't see Oyster to Reading being much different to other recent extensions, just going that bit further. Lets hope they don't try and make it a complete mess like London to Gatwick, where different fares are charged if you pass through the specific Gatwick Express platforms at Victoria...
Paul