I don't want to see any of the Civ4 features like religions and corporations reintroduced. They had good reasons to remove them. And I have no idea what "Civ5 BTS" means - it's a different game, so the expansions should be different too.
I want the following things in an expansion:
* "more of everything" - civs, techs, buildings, units etc.
* better AI
* better game performance, faster turn times (such things should be improved in patches, but expansions can help too)
* more meaningful terrain and resources, non-food bonus resources (like wood and stone)
* improved happiness system to prevent ICS
* changes that prevent other exploits and improve the game balance (can be done in patches too)
* interface enhancements, for example displaying expected losses when a city bombards the enemy, and path of units in "go to" mode
* better civilopedia - information in tooltips, more info taken directly from the database instead of text descriptions
* more charts, graphs etc.
* some fresh concepts, new to the series, instead of copying not very good things from civ4
* better modding support
* maybe some built-in mods and scenarios, but it's not very important because fan-made ones are usually better