Any show that finds a good deal of success always wants that success to continue. For some shows they run a little too long and stories begin to feel flat or forced. For How I Met Your Mother the end is well within sight as their ninth and final season is airing right now and the titular mother has already been revealed. Soon the show will end and Ted will have finally met the mother. Maybe because the show was always leading to a known ending, it's been building the circumstances for years about how he met their mother, so even as the show went on for nearly a decade you still felt like everything was leading to some payoff.
Now that it's all almost over CBS would like to have another hit comedy kinda like HIMYM, in fact a lot like HIMYM. A spin-off that sounds a lot like the original show is being planned but this time focused more on a female lead searching for her future husband. It would still follow and small group of friends in New York and may even use the same bar from the original series as a hang out spot. Current rumors even point to a possible character reveal from the new show in the final episodes of HIMYM. The idea is to provide a new show with the same concepts as the previous one but would provide a "clean slate" for characters and story development. A staple of the original show was ongoing story callbacks and a series of running gags.
I will be hard to replicate the sort of magic from the original show since a lot of what they pull off is thanks to the acting and embodiment of characters from the original cast. Throwing that away and hoping some new people can work the same kind of wizardry may prove to be impossible.