I honestly have no expectations anymore. The inconsistency in Fairy Tail is big, I agree. But Hiro doesn't mind developing Lucy's powers properly.
The Star Dress Mix is not so powerful and still has a weakness. They are temporary.
Logic 0 of Hiro Mashima.
And I hate these mix from conception. In my opinion, this is a clear waste of Seirei Mahou. Hiro could have given a second form to the spirits by making them stronger. The Star Dress would consequently be more powerful, as they receive part of the power of the spirits.
At this point there is 3 ways Lucy power developes. All of which were done at some point in the manga.
1. The most basic way is she gains more spirit keys. This is what was originally intended for Lucy power development in the beginning of the series. Would be highly beneficial to Lucy.
2. The Spirits strength increases. This is what was intended later on for her development to replace her gaining more keys.
3. She improves and develops different stardress forms. Hiro current idea of the development of Lucy's power.
Each would be time consuming to draw to write because if she gains new keys he has to come up with new spirit designs. If the Spirits power increases he has to think about new moves and possibly even new evolved designs. Same with the stardresses. I wont be to hard on Hiro as to why Lucy's magic sometimes take a back burner to say Natsu or Grays, which is because theirs are easier to draw than Lucy's. Similar to how we rarely see Erza use her armors and most of the time resorts to the same armors which are destroyed from the get go and have her go red pants.
Of all the ways Lucy power could develop I prefer number 1 and 3 over number 2. Like I think it would be much more interesting to see Lucy pair her keys up for different combinations. Like for example Aquarius would be more powerful if paired with Eridanus, Crater, Hydrus, etc other water keys because it would allow them to combine their powers for higher level attacks. Same with Sagittarius and Saggita(The Arrow) which could potential allow Sagittarius greater versatility with his arrows.
I think it is a combination of things of why we don't get to see characters potential truly explored but I think the most prominent reason is status Quo. Hiro wants characters to be at a certain level compared to others which is why we rarely see the actual power of characters with versatile magic like Lucy's or Mirajane or Erza's.