The plans, announced on Sunday, are part of sweeping changes by utilities as they look to bolster green energy and prepare for Germany's exit from nuclear power in 2022.
They come just two years after RWE spun off renewable, retail and network operations to form Innogy and E.ON split off some of its business to create Uniper.
Shares in Innogy were indicated to open 16 percent higher in pre-market trade. RWE, which owns 76.8 percent of Innogy, was indicated 8 percent higher and E.ON was seen up 6 percent.
As part of Sunday's deal, E.ON plans to launch a 40-euro-per-share, or 5.2 billion euro ($6.4 billion), offer to Innogy's minority shareholders, a 16 percent premium to Friday's closing price.
"We believe that this development will be taken very positively as the market will see a 40 euros per share takeover offer as very likely for Innogy," Macquarie analysts said in a note, raising their rating to "outperform" from "neutral".