Its a good question and I wouldnt be surprised if Delta did step up. She didnt do last season because she wanted to work on the album and with it intended for release next year - it would make sense for her to be on Australia's biggest TV talent show to promote it. The Voice Kids will be rested next year due to Nine not being able to accommodate a suitable timeslot for it this year (similarly to how they've screwed Big Brother up too - I'm confident Voice Kids will return in 2016 if Voice Adults does well in 2015).
Sportsbet currently have Delta as the most tipped to take over
Having the two huge Aussie female stars on the screen would be a massive coupe and I think Australia couldnt not watch it. Like how could that show not do well with the biggest Aussie female singers on it? Just... how? ha. And there would be a ton of rumours of them not getting on and then they would both be on each others instagram and twitter etc showing that they are and I think it would be such a great thing for Australian music in general for them to team up, if only just for performing on the show alone.
And as for the show 'stopping' Delta - it simply wouldnt. It would MAKE her career flow in Australia. Guarantees relevancy in that people know that she is back releasing music. Delta would always have to do Australia first anyway and given there actually arent all that many promotional opportunities in Australia - this is the biggest and best way to do it. And to have the support of a television network ensures she regular promotion whenever she wants it (TV syncs like Wish You Were Here on House Husbands, exhausting all TV shows like 60 Minutes, A Current Affair, Today Show and then the big performances that send you up charts on State of Origin, Logies and Voice Australia). This would definitely be a great thing for Delta to do the season on the show.
However a fifth season is something she couldnt do as she would really need to be releasing her music worldwide then and that would stop her