I feel like Princess Daisy deserves her own year. Despite being one of the major characters of the series for the past 31 years Nintendo doesn't really give this damsel the right amount of character usage in adventure based titles.
There's many things that they could do with Daisy, she can occasionally get kidnapped but be playable when you 50 percent the game so you can recruit her to go and save Peach at the end and have Daisy be a support character that sends you useful items during adventures
There's been people saying she can't be captured because she slapped Bowser but often forget that Peach had done the same with her parasol. Peach and Daisy can have similar roles but make sure to impliment their personality structure respectably because its a huge difference.
Gaming wise if these two get their own game it can be a reflection of the Mario bros games because their relationship compliments the Bro's. Daisy is Spunky and Cheerful like Mario and Peach is quiet and reserved like Luigi is.
The playstyle can be a mixture of borrowed traits from Super Princess Peach and Wario Land Shake it
Peach can have her flaoting gimick from Mario Bros 2. and Daisy can have her Double jump from Super Mario run. Each can have similar attacks but with different animations and functions when they use their trademark parasol.
This could also be a way for Daisy to have more movest potential in the next smash by decloning her a bit but still keeping her involved and similar to Peach. If Luigi can have his Mario why not have Peach have her Daisy? It will balance out the lineup aswell