I'm not trying to take anything from ETW, it will certainly be entertaining. But...
1 - In many ways, Total War's focus is broadening. The campaign map will encompasses a far greater stretch of territory, and with the figure of 10 playable factions being given, detail will likely be sacrificed on a similar scale.
A competitive title could capitalize on focus, a small land area and/or timeframe with great detail . The RTW mod Iberia Total War comes to mind on this point, having proven the rich potential of smaller scale
2 - The timeframe change is huge. Warfare in the 18th century is radically different from previous time periods and as we can already see, fan interest is highly variable. A title in classical or medieval time periods will be in demand.
3 - CA appears, since RTW, to have been shifting its priorities from gameplay to aesthetic improvement. AI development has been lagging, especially in the changes from RTW to M2. Some rival product might sacrifice polish for innovation in gameplay mechanics - this opens the door for lower budget studios to compete.
4 - Realism - demand for a more realistic interpretation of the in-game setting is evident by the popularity of many modding community releases.
For these reasons I think it makes sense that a competitor with longevity might finally arise to CA's creation