The first step for De Rossi should be to help this team finish next season in top 4. Earning a Champions League ticket would be a good start. They will need to take it slow and steady.
I agree that finishing in 4th place in the Serie A class is a good start, but if De Rossi is able to bring AS Roma to win the championship then that is an extraordinary thing even though the obstacles are very big, to face clubs like Inter Milan, Juventus, AC Milan and Atalanta, maybe for now at least De Rossi can be optimistic about his current players to be able to get Champions League tickets next season.
Sure, if they can play in UCL, they will get more income. There will be more sponsorships and more top players will join the club. Since most players dream to play in UCL, it will be easier to attract them to join if AS Roma regularly play in UCL. I agree if De Rossi may be easier to build AS Roma if they succeed to finish in UCL zone.
It's not just the financial income that As Roma will get, but the enthusiasm of the players will also increase by itself, even though there are many talented players outside, at least As Roma won't make the mistake of buying players so that the game remains as good as last season, even though it only finished in the place. 6th place at least playing in Europa is much better for next season, and it looks like De Rossi is still able to improve next season's ranking with several additional players in several lines.