Now i'm interested in how "End of Time" would have worked out
Crashes through the sky light and then?
How would the TARDIS have been damaged?
RTD wasn't anywhere close to writing "The End of Time" when Tennant turned down Moffat's offer for series 5. They were filming the River Song two-parter at the time, so while RTD knew there was going to be a year of specials, he had no idea what those specials were going to be.
If Tennant had wibbled and decided to stay on for series 5, if there was an "End of Time," it would have been vastly different -- and RTD would likely have written his final special as a lead-in to whatever Moffat had in mind.