According to a report by Sponichi, the Eagles would like to convert Kleenex Stadium into a natural grass field. If talks with stadium owners (Miyagi Prefecture) go well, construction (estimated at over 100M yen) to swap out the artificial turf could take place after the current season.
General maintenance costs are also expected to rise (due to the weather in Sendai), but Rakuten is not as worried about the costs as they are about providing fans (an MLB like experience) and players (less strain on the body) with the best all-around environment.
There are currently only two NPB fields that have natural grass in both the infield and outfield: Hiroshima's Mazda Stadium and Orix's Hotto Motto Field Kobe. Koshien Stadium does use natural grass, but only in the outfield (the infield is all dirt).