Some Re:Zero fanfic ideas, crossover mostly

Big Sonic 5: Some general ideas(spoliers S2 anime and after with warnings):

Some more general stuff: Thanks to Subaru being just some random guy, there is a ton of license to fudge what he would know and what elements to take, like much of the Sonic main cast has social media, even Knuckles, so info can get out there from there, or they mention it to the media(would expect like, Tangle to be specially talkative). You can have Subaru do literally nothing out of the ordinary or like, explore random islands pre-summoning(definitely have extreme gear if the latter). This could even be a 2-parter with part 1 being pre-summoning and part 2 being post-summoning. You can even have him learn techniques such as a weakened spin dash(Cream, a 6 year old, was able to learn it), Advance 2 midair tricks(Also Cream), Rouge heal from Sonic Battle(if this part of Battle is cannon, states technique was part of her special training), wall jump, any of the Forces avatar wire stuff, or possibly some of the more grounded things Amy does with the Piko Piko hammer in Games such as Adventure and Advance(ex. Hammer jump). (Gunplay if you're feeling extremely edgy) (Probably limit the amount of these if any for easier writing and/or have them focus on transversal that would be fairly useful in Sonic world, but would be very overkill in Re:Zero world)(Advance 2/3 boost mode could be cool in combination with midair tricks and a hammer similar to Amy's with the running version of hammer jump if you have hammer jump's height depend on speed)(Could also have him have shoes designed for grinding and him treating stuff such as vines, ropes, and the tops of houses as grind rails)

If Subaru retains some amount of info from bad futures, then that would include info about the Re:Zero world thanks to how they all would inevitably go past his summoning date(unless the bad future stops his summoning) so retained info should probably be limited to whenever time is destroyed.

Stuff that can be done during the story: Sol Emeralds do have some sort of warning about upcoming terrible events, including other dimensions because Blaze was able to listen to their will and go to Sonic's world to prevent one, but they "are not... chatty", so Blaze with the Sol Emeralds can appear shortly before any Great Disaster, trying to figure out what's happening, like you could have her appear in this Arc 7, but at the other side of the country or something and only getting into the plot later. Blaze is also able to use the Sol Emeralds to restore memories, but I am unsure if this only applies to those who, before memory loss, interacted with the Chaos Emeralds, Sol Emeralds, Blaze knew personally, all of the above, or none of the above(probably have Subaru seeing Blaze speed by, on the news, or in a video on social media if you do this).(If you go with the Generations recommendation, then he could have met her earlier)

You can also limit the possible times Subaru is summoned from by saying it requires reality to be relatively weak(whether destroying time or warping reality), with possible times including Sonic advance 3, Sonic Rush, Sonic 06, Sonic Generations, within the vicinity of the Phantom Ruby during Mania and Forces(entire war on latter with heavy use of Phantom Ruby), and the entire Sonic Channel/Otherworld Comedy (2023). (Other times to isekai characters with these rules that probably wouldn't want to use for Subaru include eras and locations that Generations levels take place in, whenever Solaris comes from, little planet from probably whenever(kidnapping extremely small amount of residents/vacationers over time if so), near the Warp Topaz when in use(especially when overloaded, could say that Dr.Starline was briefly in Re:Zero world and possibly others before going back if you choose this), during the events of Sonic Prime, on Onyx Island when it briefly appears, and possibly in the vicinity of anyone using Chaos Control)

I'd recommend Subaru trying to do cool poses after beating a tough situation, based on Big's posing after catching Froggy while being a random civilian, it's normal there.

Couple worldbuilding things that can be added to the premise:You could also say that Ring energy, Chaos emerald energy, and Sol Emerald energy are similar to mana, but different(Chaos and Sol energy are noted as being similar and different, with both super forms using rings as fuel). Earth life energy(Lost World) would probably be identical to mana. You could also have Chaos Drives be very similar to mana crystals and even have Echidna make some very similar devices to those back in the day. For this, you could also say that Badniks run off of the mana of the animals that power them.(would also add Power Cores to this system, probably) Note: Knuckles heal from Sonic Battle says that the user uses earth elementals to heal their wounds.