Very likely you have already noticed it (I have the game just from few days), however I wanted post what inside the game seems to suggest the idea of a planned 30th character:
- In the random filter page for the characters, in the lower right corner appear to be just an empty space for including another name;
- In the movie victory pose section, the last row ends with an empty place suitable to be reserved to 3 new animations;
- In the movie lose pose section, the last row ends with an empty place suitable to be reserved to 1 new animation;
- In the statistic section, the list of characters is formed by three pages, the first showing the characters from the 1st to the 10th position, the second one from the 11th to the 20th position, the third one from the 20th (that so is repeated) to the 29th position;
- Lastly there is always the presence in the character selection of an "useless" second dice.
The date of the announcement of the next character for the Arcade (while DoA5U wasn't still full released) should make it obvious, however the elements listed over should dissipate any doubt about the later release of the character like DLC.
PS: In the random filter page for the stages there is an empty place suitable to include three new ones: maybe they will be released sooner or later? According to me a new stages together with the next character is possible.