Opinion piece: What system would you use to run an Isekai and why?

I was thinking about using Tails of the Valiant or D&D since Kobold Press made an Isekai'd book that includes things like the summoning spell used in some of the Isekai stories

I would use F.A.T.A.L. to run an isekai.

Fabula Ultima or Infinity's Edge. FabU is awesome for how well it maps the genre and IE is mechanically geared towards Isekai.

For the vibe, I'd say BESM. It's meant to play out like anime.

For starters, I find it difficult to answer the question as written because it's unlikely I would GM isekai, and there's a narrow overlap between isekai kinds that I was sufficiently impressed by to consider, and systems I'd want. But for the sake of this question I will treat both of those funnels as more permissive than they actually are.

• For something very nerdy, where the PCs are isekai'd from a high-tech world to a low-tech world and use their knowledge to great effect, there's GURPS. It has rules dedicated to differences in tech levels, having knowledge/skills of different tech levels, inventing stuff, and raising the local tech level.
• For something like Perks of Being a Villainess, where there's less (but not zero) focus on hard science and engineering, and more on social engineering (like intrigue and marketing), the selection of systems is wider. There's of course GURPS with its Social Engineering, but also things like Storyteller (nominally socially-varied system), FATE Condensed (Social Conflicts!), OVA (anime-oriented, reasonably fine-grained characterisation), or I guess QuestWorlds (not too impressed, but it does give a lot of freedom in how one characterises one's PC).
• For something very hack-and-slash like Shield Hero, there's Fabula Ultima (which I never played, but which looks like it focuses on a bunch of combat abilities that you either have or don't, gear tiers, &c.). Or maybe Gates of Krystalia, which I also never played, but which is marketed as The Isekai RPG (and also seems to have a combat abilities focus, but this time using cards instead of dice) - might be an interesting expermient to try.

Will your game be leaning heavily on the genre of anime? That's the easiest to answer, as suggested by earlier posts: Fabula Ultima, Big Eyes Small Mouth, OVA, Break!, or even anime-skinned D&D 5e.

I don't think you need an exact corollary to an isekai summoning spell to play such a game, anything that recreates the anime feel will do.

Or are you more interested in the idea of having some 'modern' abilities and rules, and then seeing how the system applies when you insert these contemporary features into a fantasy world. If so a game system with a wide set of abiities and skills, or broader definitions of these so that you can flavour them accordingly, might be more suitable.

I had to look up the term Isekai. You could just aswell have said "fantacy". The real question, you have to ask yourself, is what story is it you want to tell? Pick the system that best supports the kind of story you want to play

Heroic fantasy, Marvel combined universe style? - go with D&D5e
Heroic fantasy? - Daggerheart or Pathfinder
Gritty fantasy? - Dragonbane or Warhammer Fantasy.
Something in between? Savageworlds

But if you have a sourcebook or expansion book for a system that supports the story you want to tell, then go with that.

Not just any fantasy. The fantasy of a character from one world getting into a different world, usually either by way of reincarnation (how the main character becomes Deborah in Perks of Being a Villainess) or summoning (how the main character in Shield Hero arrives in the adventure world), or some other means (how the Necromancer ends up in our world in Dead Mount Death Play).

Break!! handles Isekai pretty directly...

If I were to personally run something, I'd probably go lighter weight with FAE, Risus, Tricube Tales, Freeform Universal, Cortex Lite or the like.

Being familiar with such, I'd use BESM or BESM d20, but really, any system can be modded to handle it. I once ran a d20 Modern game where drow had invaded our world and caught everyone by complete surprise. It had visitors from other dimensions in it. Or, hell, for that matter, use Rifts.

I would use a system that spans contemporary, sci-fi, and fantasy. Probably one that uses points for character creation (Gurps, Hero, D6 are the ones I know). I would have all players create characters based on the original character. As most Isekai gain some advantage due to the translation/reincarnation I would then let then either add a new background (growing up after reincarnation) or a new race (transforming as part of the translation) start with some bonus if they continue as the original character.

To make it fun you could mix all the various types of isekai where the reincarnated characters 'discover' the other recent arrivals. The original characters could start with the same origin world/time or completely different.

I would probably use Genesys (literally generic setting version of FFG's SWRPG if anyone hasn't heard of it) since it has the advantage of working with differing settings built in, tech levels, magic, so on and so forth so I'd have more options for what kind of world the players would be dropped into readily available. Alongside having social and narrative mechanics built in by default which would help me work with the themes of being trapped far from home having to setup up shop in a completely foreign world, focusing on the initial building blocks of food and water to first interactions with the world's people, learning, diplomacy, building a place for themselves there, maybe learning magic if it's a world with magic, interacting with crazy inventions if it was a steampunk world, etc.

It would work well for the narrative and character focused game I'd want to do alongside my own liking for the FFG's Star Wars combat system that carries over to Genesys.