First of all, holy heck is the Mindscape stuff super cool! The flashy presentation style, with dialog and camera swooshing and all kinds of cool effects everywhere is incredibly fun. I loved the Moonreaver fight, and the Ultimate Inazuma sequence is an incredible reward for succeeding, but there's a few glaring issues.
Chief among which is that I almost completely missed it!
For whatever reason, I happened to just never see the end of level portals in two playthroughs. If I had to guess, I'd say it probably happened becasue I have a tendency to not worry too much about collecting rings when playing in superform, especially near the end of the levels. I ended up turning Aether's castle upside down looking for a hint about what to do- and I only did that because I happened to know about mindscape for out of game reasons. In the end I only found the answer from people discussing it on discord. There really should be an npc in the portrait room or something hinting about how to get the different endings.
The Ultimate sequence has similar issues. While I figured out the meditation hint pretty quickly, the fact that others didn't implies you really should have a dialog box explaining things pop up after a few seconds or something. I also struggled a bit to figure out exactly how to destroy the first Phantom Eye. This the player's first time being Ultimate Inazuma, I didn't even know I could shoot lasers, much less the extra inputs I have to do to do it in the air. As a general rule, you should only be asking the player to do one new thing at a time.
I also fear that while I enjoyed the heck out of Mindscape, my tendency to play with an infinite lives cheat may have influenced things. I died quite a few times against Moonreaver, and I'm certain that if I'd had to replay the entire Mindscape section multiple times, it would have worn out its welcome very quickly. Nothing ruins a cool cinematic sequence faster than having to play it over and over again. The fight would have still been plenty difficult and exactly as climactic even with a renewable 1up sitting outside the arena.
I hate having to write so much negativity here, because this is genuinely one of the coolest sequences I've ever seen in SRB2! The Moonreaver phase is exactly what I want out of a difficult boss fight in this game- a frantic dance of learning tells and trying to edge out openings. Most of the damage I got out was with Aether- which I choose to interpret as a ludonarrative metaphor for her dragging Inazuma out of his despair. Please do not correct me. =w=