RIZIN should worry about themselves and I doubt TOP would care to work with them, I don't think ROAD is even really committed to the whole alliance thing.
Having multiple promotions in Korea to choose from is probably better for the fighters but it really hurts the division strength of both. If they were to combine we could have something along the lines of (not including possibilities that may be possible through TOP's co-promoting with Kunlun Fight):
WMMA - Ji Yeon Kim, Jung Eun Park, Ye Ji Lee, the DEEP JEWELS fighters that often fight in ROAD, Xiaonan Yan, Yoo Jin Jung
Flyweight - Nam Jin Jo, Min Jong Song, Farmon Gafarov, Mikihito Yamagami (if he is even still fighting), Kai Asakura, Kyu Sung Kim, Yeon Soo Kang, Joo Hwan Kim, Jung Bum Choi
Bantamweight (the most interesting division, tons of talent) - Kwan Ho Kwak, Min Woo Kim, Yoon Jun Lee, Soo Chul Kim, Yuta Nezu, Heili Alateng, Ik Hwan Jang, Zhikang Zhao, Je Hoon Moon, Shoko Sato, Hyung Geun Park, Jung Hwan Ahn, Won Jun Jang, Han Bin Park, Myung Gu Kim, Jae Hyun So
Featherweight - Soo Chul Kim, Yoon Jun Lee, George Roop, Mu Gyeom Choi, Thiago Silva, Marlon Sandro, Seo Doo Won, Young Gi Hong, Hiroto Uesako, Tuerxun Jumabieke, Hyung Soo Kim, Min Gu Lee, Seung Woo Choi, Young Gwang Choi, Sung Won Jo, Sung Hwa Han, Dong Gyu Kim, Won Il Kwon, Shoko Sato
Lightweight - A Sol Kwon, Albert Cheng, Kyung Pyo Kim, Bruno Miranda, Kwang Hee Lee, Shinji Sasaki, Won Sik Park, Seung Yeon Kim, Chan Sol Park, Jung Min Kang, Gyo Pyung Hwang, Michael Ahn
Welterweight - Kiyoshi Kuwabara, Batmunkh Burenzorig, Sung Chan Hong, Sung Won Son, Han Seul Kim, Takenori Sato, Jung Hwan Cha (currently ROAD's MW champ but the title fight was his only MW fight, normally a WW)
Middleweight - Young Choi, Dong Sik Yoon, In Jae La, Jung Hwan Cha, Riki Fukuda, Minowaman, Eun Soo Kim, Jae Young Kim, Jae Young Ahn, Sang Soo Lee (he fights everywhere MW and up)
Light Heavyweight - Zilong Zhao, Roque Martinez, Doo Hwan Kim, Sang Soo Lee
Heavyweight/Open Weight - Hyun Man Myung, Mighty Mo, Aorigele, ROAD is pursuing Jerome Le Banner and Peter Aerts for their upcoming Open Weight Grand Prix, Hong Man Choi, Mu Bae Choi, Carlos Toyota, Da Un Jung, Jun Soo Lim, Roque Martinez, Sang Soo Lee
Working together would give them one of the best Bantamweight talent pools in the world and give them solid Featherweight, Flyweight, and Middleweight divisions and a few promising female prospects. Then they have the Heavyweight division with some aging stars that can still pull a lot of viewers.