I wouldn't mind having the next game in the series occur before Skyrim or even Oblivion.

After. Move forward. Exploring past periods in Tamriel's history is better suited for spinoffs, not numbered titles.

Numbered games should advance chronologically and contribute to the overarching story. Spin-offs can take place whenever.

Suggestions for future TES games can be discussed here: http://www.gamesas.com/topic/1613350-official-beyond-skyrim-tes-vi-90/

There's also a thread for locations/settings, which includes timelines: http://www.gamesas.com/topic/1609912-tes-vi-location-and-setting-speculation-35/