For me, personally, when I subscribe to services the major incentive is improvements of the existing service. I subscribe to YouTube Red to not be bombarded by ads on mobile, and to listen to music more easily. The features added aren't gigantic, nor do they alienate people who don't subscribe, but they're good enough to subscribe to and enjoy.

The new subscription service in Dokkan should be used for improving the game, and not giving subscribers small increases in existing rewards.

To expand on the latter point a bit, the rewards given will not be monstrous (they can't be from a business perspective), and the value given is not as meaningful as improvements being made for the game.

Bandai won't allow the subscription service to give players a ridiculous reward for subscribing. Two reasons : it will undercut the revenue of the existing shop, and it will alienate free to play players who don't subscribe (the first point being the most important). Furthermore, I don't think the increased rewards, whether it be a an increase in DS monthly, or the addition of a daily free pull, will lead to greater enjoyment of playing the  game. Just look at Naruto Blazing and Pokemon Masters as examples of how boring a game can be with greater rewards. Naruto has free daily pulls, and Pokemon has been giving out one free single a day; both games are boring and are losing players daily.

Now for the argument of using the subscription service to improve the game.

Dokkan has been out for four years, and I've played it for release week. There are a few key improvements that could be made, but would likely require resources and interest; this subscription service may be the catalyst for making these changes.

For people subscribing to the service, get rid of STA refresh time. The idea being that people are paying a subscription fee in order to enjoy the game more, let people play it more often. In the case of the WT, since its competitive and includes F2P individuals just leave the STA refresh enabled for all as usual. In the 4 years of playing this game, I can count on one hand the number of times I've used more than 10 DS to refresh STA to complete events, most people stop playing when the STA runs out.

Improve the software and business model of the game.

One notable performance item is that when there are a lot of events active, the event section and slider are a lot slower and less responsive. Because of this Akatsuki implemented the Portal of memories, for the reason that it allows people to play old events, while simultaneously restricting people to avoid too much server strain. With the money collected from the subscription service they should really try to optimize their software and their services. The idea being that they now have a revenue source from people who want to see the game improve, so they should use a large sum of that money to optimize the game.

From the business perspective, they should really look into getting more resources or restructuring exiting ones. Its been way too long  since a Quest stage update, Strike Character EZA or inclusion of new Super Strikes (RIP Piccolo Jr.), expansion of Heroes v. Villians, Awakening of Gem SSRs, etc.
I would imagine this is mostly due to a lack of resources, and because other events may take priority