I wouldn't say Timeranger "flopped." Its toy sales were lower than usual, but it's ratings were OK. It turned off some younger viewers, but the longtime fans liked it, and Toei seems to think of it as an accomplishment. It couldn't have been all bad -- it's not like it died a pathetic disgrace like JAKQ.
As for being overshadowed by GoGoFive in their versus movie, well -- all of the versus movies up until then royally ripped off the previous team. Only Red, maybe one of the heroines, would be focused on, and that couldn't really be done when GoGoFive was a whole family.
As for there not being a Gaoranger VS Timeranger, well, it wasn't needed. I do feel like Toei tried to treat Timeranger with some respect, because bringing them back for a Gaoranger VS Timeranger or one of them in Gaoranger VS Super Sentai would have ruined the bittersweet finale. They were going back to the future, never meant to return. (Otherwise, what's to stop them from popping up every now and then to play leapfrog with Tatsuya?) The finale would have lost any of its impact if in, say Gaoranger VS Super Sentai, Ayase inexplicably returns to 2001 just to be bonked in the head with a G-Phone and having to teach Gao Blue about sentai. Yes, there was a time where Toei cared.
Plus, Toei wanted to do Gaoranger VS Super Sentai instead, which was better than any sort of Gaoranger VS Timeranger movie would have probably turned out.