Been listening to Brandon's Hard Boiled Fury, and y'know, despite most Mario v Sonic matchups outside of Mario v Sonic itself kinda not being good, I genuinely really still like Bowser v Eggman. Their personalities bounce off each other well, as characters they contrast well, they have the capacity to both have a really fun 1v1 fight and a really fun army fight going on in the background between their armadas and infantry (I'd probably leave Metal Sonic & Bowser Jr out of it though, so they don't tip the scales too much and their other infantry can have a spotlight), and the dynamic of Bowser being an uber-durable tank against Eggman's versatility and intelligence is a really fun idea.
Give Bowser some of his whackier weaponry and equipment, Clown Car, Tennis stuff, maybe some M&L RPG abilities, and hell, you could even reach into his old sorcery for some fun, and Eggman could have his various forms of his Egg Mobile, Big Arms, Egg Dragoon, Death Egg and Death Egg Robo, and with the Phantom Ruby could probably summon more of his specialized equipment as well (or you could just have them stored in his Death Egg, which is very much so standard equipment lol) to keep his versatility and machinery constantly evolving throughout the fight and introducing new shit for Bowser to deal with.
The battle can vary depending on what equipment you deem "standard" for Eggman, as I can understand the arguments for not allowing the Time Eater, Egg Salamander, or Egg Wizard as despite being some of Eggman's strongest equipment that would definitely net a win against Bowser, they were only ever used once, or needed external help to be used, which would open up allowing Bowser the Dream Stone, which is a little copey and throws a wrench into things (especially considering that Dreamy Bowser is essentially one big anti-feat, and there are valid debates against it not even being 2B in an ongoing CRT right now actually). So it'd probably be best to keep it to their best standard equipment, and maybe a handful of special equips that have good reasoning for being pulled.
I do think the fight would end up in Eggman's favor though, as even his base machinery can reach up to 4A which would let him stay in the game for a long while, and without the Dream Stone or Paper Mario feats, Bowser's best stats come off of his 3C scaling from the Grand Stars, and considering those are equipment only used in two games, it should be totally valid for Eggman to have the Phantom Ruby, since it was his main equipment in two games as well, and considering the full power of that stone is universal+, I'd say he'd be able to pull through in the end. There's also the blatant wincon of allowing him the Chaos Emeralds as well, he has used them multiple times and they could definitely power a machine that could finish the fight in the same way that the Phantom Ruby could. It's a lot closer than most MUs, and I think it could be really fun. It's one of the only Sonic MUs on the show I'd care to see and is one of the only Sonic character runback that I think is a good idea (especially since Eggman's last MU was a S1 episode and he's had so much more since, and Bowser's last outing.... ******* sucked). I'll be pretty happy if it's anywhere in Season 9.
Anyone have baller ideas for how the fight could end? Simply saying "The Phantom Ruby/Chaos Emeralds would give him the win" is fine and all but there's always a sick way to have a fight get finished. Getting his shell drilled into with the Egg Dragoon's drill? Have him weakened enough by the Ruby that the Death Egg's constellation destroyer beam could finish him off? Banish him to Null Space? Create a pocket universe, BFR Bowser to it, then destroy the universe with him within it? There's plenty of opportunities here with an arsenal like Eggy's and weapons as versatile as the Phantom Ruby/Chaos Emeralds
(Note I am not utilizing any of Bowser's feats from the Paper Mario games because I don't like doing that shit, and I'm pretty sure Death Battle doesn't composite them either? Even if I did include PM stuff though, there's a chance the Pure Hearts might not even guarantee a victory anymore since they might lose their 2B rating as well soon based on the same CRT as the Dream Stone thing.)
(random uncalled for text wall I know, but idk I got randomly passionate about this fight lol)