In the chocolate heart it's written: 桜高軽音部 これからも よろしくね ❤️ (Sakura Kō Kei On Bu, korekara mo yoroshiku ne ❤️)
This can be translated into: Sakura High School Light Music Club, from now on, please take care of me.
Despite being a normal thing in Japan a novice student ask her/his veteran to take care of her/him in the sense of instructing the younger one how things work at the school (like Azunyan did to Yui in her first day as a member of the Light Music Club), what we have here, basically, is Mio-chan using the relation to her veteran as a mean to propose to Megumi, as if asking her on a date. And she's giving Megumi those words written on a chocolate heart, showing that Mio has a lot of feelings for her too. Last not least, Mio is using the same maid costume she used on that night she tripped on the bass wire, showing her panties to the audience where Megumi was at that time (and what triggered her passion for Mio-chan) - that is, this speaks volumes by itself.
On K-ON! fandom we can find more details about something near a relationship between Mio & Megumi (some people in the studio even wanted to make clear that they were dating, but in the end they feared showing that Mio & Megumi were in a relationship could affect sales...), like for example when Mio tripped over the bass wire and fell, showing her panties to the audience, this gave rise to a (to a certain extent) secret crush on Mio in Megumi. Another interesting lore we find there is that Megumi was commonly seen in the Light Music Club, watching them play - even though this was not mentioned in the original series, these details were included in the remastered version that came out on Blu-ray, along with some correction like Megumi Sokabe being placed only one year ahead of them, instead of two as said in the anime.
