The plant person from Mebius 40 was portrayed by Nobuhiko Tanaka, whom I last saw in Ultraman Nexus as Yu Kirasawa (aka the Illustrator). Kirasawa usually gives orders to the Night Raiders (the show's monster attack team). He is super intelligent, although his job seems somewhat similar to Ishibori's (see below).
Tanaka is the second regular from Nexus to appear as a guest star in Mebius. The first one is my super favorite scumbag reporter, whose first appearance is in 28. Kousei Kato (Mitsuhiko Hirukawa's actor) was in Nexus as Mitsuhiko Ishibori (ha, same first name). Ishibori is Night Raider's technical, tactical, communication guy (think Teppei's and Konomi's job). It's sad that I like him more as a bad guy in Mebius.
Ayame Koike, the little girl who played the younger Marina in 23, was also in a few episodes of Nexus. And may I say that I love this girl? Unlike some other child actors I've seen, she can act.
Takenaka, Aya Jinguuji's grandfather, was portrayed by Kenji Sahara, who was in Nexus as Tougou, one of the heads of TLT (the head organization of Night Raider). Sahara-san was also in Ultraman Max and other various Tsuburaya productions.
Masami Horiuchi appears for perhaps one minute in the Mebius movie as a mayor named Matsunaga, which is probably some kind of a homage to his character from Nexus, Youichiruu Matsunaga. (The Mebius character is, to my knowledge, not the same as the Nexus character; they only share a name.) Matsunaga supervises the Night Raiders and answers to Tougou. His ringtone in the Mebius movie is Nexus' Night Raider's theme. (And I was so happy that I totally recognized that ringtone without having to look it up first.)
Hopefully I haven't missed anyone. Gotta love Tsuburaya for keeping these people in the franchise.
ETA: Doh! I really did miss someone. Keiko Goto, who played a Night Raider member Shiori Hiraki in Nexus, appears briefly in the last few episodes of Mebius as the mother of a little boy. (I would rather say that she doesn't have enough screentime for me recognize her, but that would be lying. ;)
Not human, but the dam in Mebius 46 is figured prominently in Nexus as the base of TLT, Fortress Freedom. The Night Raider headquarter is underneath the water behind the dam.
As stated elsewhere, I love Nexus (first series I bought R2 for). It's dark and very serious. Can be somewhat slow (sometime takes four episodes to defeat one monster), but the human drama is wonderful. And unlike other Ultra series (including Mebius), it's rather continuous, more like the new kamen rider series where they don't really have self-contained episodes. If anyone is interested, TV-Nihon has subbed the entire series.