It's occurred to me now that by the end of season four or at some point during the course of the series, we are kinda expecting that Layla and Nabu are going to be married as well as Bloom and Sky. By that point, image-wise, married characters are usually not very popular with young demographics and the best way for a producer to deal with that is to smoothly transition out the character. Also, we have to remember that once most people are married in real life, their priorities tend to rearrange themselves to fit their current life such as Layla taking the throne of Andros/Tides.
Some more familiar examples are (in Pokemon) when the main female trainer is replaced by another female trainer such as May replacing Misty and Dawn replacing May. Another example was when they replaced Brock with Tracy in season two, I think, because that was when Pokemon was going global and the producers decided that Brock would seem racist (because he's black) to American veiwers.
There's also the issue of having one too many Winx girls. If and when Roxy becomes an official member of the Winx Club, they will be seven girls in total. That is already a lot of characters for one show. Then there is the expectation that she will get a boyfriend and maybe even a bonded pixie if the producers at Rainbow are that generous (even though they have Pop Pixie in the works). That's already seven main girls, seven main guys and seven pixies. There's also her dog Artu to think about, too.
I don't necessarily mean that Roxy is going to replace Layla or Bloom as soon as they marry and it is still very possible Roxy is only a tagalong character for the season like Flora's sister Miele/Rose was for her episode. Rainbow cannot really prolong Bloom or Layla's marriages for too long or else their audiences become unsatisfied with the show.
Can this possibly be what's going to happen?