To keep this largely about Serena, honestly, she really should have been given an actual goal a long time ago. She may have some promise with the whole Pokevision thing, but we still don't have anything concrete yet, especially when they were setting May up to be a gym battler like Ash yet she went for Contests instead by the time Roxanne came around, or how Brock was mentioned to desire to become the world's best Pokémon Breeder, yet with the exception of the Suzy-related episodes and possibly the Stantler episode in Johto and his raising Togepi as an egg, we got absolutely nothing relating to his goal (heck, even Misty in Kanto had more focus on Water Pokémon Master than Brock did in his entire time as a main character, and even you can't argue that), and Johto was even worse as Gold and Silver actually introduced the breeding mechanic, something that would have given the writers a perfect opportunity to showcase with Brock, having him mate several of his Pokémon and getting an egg in return, yet they never did. I mean, honestly, in the time it took up to now with AG, May had already done her first Pokémon Contest and made that into a career choice, possibly was on her way to doing the second contest. And we also need her to have more to her than just having a crush on Ash, especially when, honestly, her even knowing Ash conflicts with the canon that Misty and Gary were the only friends Ash had (as Molly was more than an acquaintance) prior to and during the first stages of his journey. And besides which, other than her crush, she's effectively a blank slate, not much we can deal with to actually get glimpses into her character, and we may even get a Hina characterization if this keeps up. And honestly, she needs to actually train her fire starter at least, if not actually capture some Pokémon so she at least has some point accompanying Ash and co. And they definitely need to expand on her personality. Even May, who didn't even care about Pokémon training in the beginning, at least made an effort to train her Torchic and at least capture some Pokémon, even before Roxanne. Heck, even Brock, despite not doing much regarding his actual goal, and I would argue contributed the least to their goals among the main cast members (Max doesn't count, as, let's face it, he can't train even if he wanted to since he's underage), at least captured some Pokémon and attempted to train them. What has Serena done lately regarding, you know, actual training, never mind actually attempting to capture Pokémon? As far as personality, like I said before, we need more expansions. Heck, maybe surprise us and have her avert the Hina stereotype similar to Princess vs. Princess with the infamous Fiorello Fangirls scene. One thing I think is pointless to argue about her, though, is her only being a love interest or only a cute face. Not because I want it, but because some of the people complaining about this case haven't even paid attention to the fact that this trend started with Advanced Generation. Heck, the ex-director even made that very clear back in 2010 that, at the very least May and Dawn, if not Iris and possibly Serena were specifically created for sexual fanservice purposes in a cynical attempt to get kids interested in them simply for being a cute face rather than, you know, actually having a character, so even if it is a flaw, there's nothing we can do about that especially seeing how even when we did speak out against it, they ignore it, even when we actually mail them.
And just a final word regarding Misty before we stop talking about her, 1. Misty was already established to be a gym leader in the seventh episode of the series (and as DBZ pointed out, several other episodes after that), meaning the PIA thing was pointless (not to mention why didn't Misty return to the gym sooner like, I don't know, immediately after the events of The Misty Mermaid when she more than earned her sister's respect and thus actually could become it if she really wanted to). 2. If she genuinely wanted to become a Gym Leader, why would she be upset at all regarding having to take over Cerulean Gym, regardless of leaving Ash or not? Was May upset at having to leave Ash at the end of AG to pursue her dream at Johto? No, if anything she was way too eager to leave him. Same with Cilan and Iris when they left. And Brock, twice (well, technically four times, but I'm meaning intended permanent departures here, so Orange Islands and DP since OI was intended to have him permanently leaving the group), and heck, Tracey as well. Probably the only ones even remotely upset at having to leave Ash were Misty, Max, and to a lesser extent Dawn, and even the last one got over it pretty quickly and obviously didn't get upset about her goal. And 3. Had she actually liked her job at Cerulean Gym, she would not have deliberately tried to constantly delay her return in her second reappearance, period, and certainly not wait until Daisy basically threatened to give away badges to even consider returning to her duties, so no, she definitely didn't like running the gym at all. Oh, and Misty also used the first opportunity after Daisy took back control to head to Hoenn, leading to the Mirage Kingdom arc, showing just how little she really cared about having to run the gym. Had she actually liked running a gym, in other words, she would have been just like Jason Fox from FoxTrot relating to school (loves school, hates breaks even when it means more freetime with his friends).
And for the record, since you mentioned that wasn't a "real" definition of a Water Pokémon Master, I guess prior to DP, there wasn't a "real" definition of Ash's Pokémon Master either, especially when it was constantly contradicted (like collecting all Pokémons, beating a league, becoming unbeatable, winning the Battle Frontier, all of that). If you are going to claim winning a league (which, BTW, is conflicted with his winning the Orange Islands League, yet still continuing his journey) is what makes a Pokémon Master, then the stuff DBZ Fan mentioned equally applies to Water Pokémon Master pre-Johto, especially when this is stuff ACTUALLY STATED AND SHOWN on the show, not simply what we want it to be. Had we been arguing for what we wanted instead of what the show explicitly showed and stated, we'd have been stating Ash has been replaced by Johto since he achieved the rank of Pokémon Master in the Orange Islands since that's what we wanted. Or we'd be saying Brock has bred Pokémon and has lots of baby Pokémon since that's what we want. Or, heaven forbid, that May accompanied Ash to Sinnoh for contests in that region instead of going separate ways since that's what we wanted. Oh, and had Misty truly had an inconsistent goal, she'd be like Lola, Brock's mom, who basically did several hobbies before going for Pewter Gym Leader for no reason other than simply hobby material, with no overarching goal in mind. Even in Kanto, her desire to capture and train Water Pokémon and to get stronger was the most consistent thing about her, so no, she did have a consistent goal even before Johto.
Sorry if this came across as a bit long, but it needed to be said. At least I managed to insert Serena as the first paragraph above to keep it on topic and avoid spamming.