More Than Just Friends?
Sailormoon, as a fandom, has as many pairings as it does characters. Whether it be official couples (i.e. Usagi and Mamoru) or fan pairings (i.e. Makoto and Ami), the possibility for two characters to be interested in one another romantically is nearly limitless. So, if such a thing seems to be so common, is there anyone to say that Chibiusa and Hotaru are not just friends? There are instances (which I will list below) that could be taken as such, but I will leave the final verdict to you, the viewer.
First Time:
In the manga, during the final battle between Pharaoh 90 and the sailor senshi, Hotaru realizes she holds the key to saving thr souls of the four inner senshi and the crystal of her best friend. She separates herself from her possessed body, and returns the souls of Mercury, Mars, Jupiter, and Venus, before flying off to return Chibiusa's life. Appearing in a semi-transparent form, Hotaru offers up Chibiusa's silver crystal, allowing the crystal to return to its rightful owner. This saves Chibiusa's life, letting her awaken and see her friend for the last time. The two exhange some final words. Hotaru is quoted as saying, "I was able to save you, Chibi-Usa-chan. I'm happy. It's good. We're both girls, but it's strange. It must have been fate that we met." Chibiusa agrees, and the two girls hold hands before Hotaru's spirit is forced to fade away.
Interpretation: Hotaru states she finds it odd that it seems her and Chibiusa are connected by fate. The two are total opposites on the outside, but inside, they are very, very alike. Chibiusa understands this, and agrees with Hotaru. I don't believe Hotaru means that she thinks it's strange that she feels so strongly for Chibiusa, despite them both being girls.
Second Instance:
Another instance in the manga occurs in the first volume of the Stars arc. While Chibiusa, Hotaru, and Usagi are walking to school, Usagi stares at the two young girls in front of her. She comments, "You two are good friends...Holding hands at this unearthly hour...", to which Hotaru responds, "Well, don't you hold hands with Mamo-chan?" Usagi is very irritated by this comment, and breaks through their linked hands in a rather childish way.
Interpretation: Hotaru's line can be read from multiple angles. You could read into the line, and attempt to say Hotaru is comparing the bond she shares with Chibiusa with the relationship between Usagi and Mamoru (i.e. since Hotaru feels the same way about Chibiusa as Usagi does Mamoru, why shouldn't the two girls hold hands?). Another way to interpret the line, which I believe, is less philosophical and more literal. Hotaru has probably physically seen Usagi and Mamoru hold hands. So, since Usagi and Mamoru can hold hands, why can't she and Chibiusa? The difference in this interpretation is Hotaru isn't comparing her feelings for Chibiusa against Usagi's feeling for Mamoru, she's merely commenting on the fact that since Usagi can hold hands with someone that's important in her life, why shouldn't Hotaru have the same luxury? Hotaru doesn't assume that you have to be in a romantic relationship in order to hold hands with someone. It can be disputed if Hotaru may actually be poking fun at Usagi (indirectly stating Usagi and Mamoru's relationship is only friendship just like her friendship with Chibiusa). This would make sense as to why Usagi reacts so annoyed, or it may have been Usagi intrepretating Hotaru's line as such when Hotaru did not mean it that way.
Neither of these events happen in the anime, but the two are often found holding hands.
There is no other concrete evidence I can recall that would suggest anything other than friendship for these two girls. It is obvious Chibiusa and Hotaru share a very close bond, but I personally do not feel it goes beyond friendship. Chibiusa also has romantic feelings for Helios, even after she discovers Hotaru has grown since her rebirth. However, I will not force my beliefs on anyone, and if you do believe these two do have romantic feelings for one another, then to each his own.