Wow, thank you very much! That's extremely helpful.
When no me bré happened to say 'Willkommen bei LEO' in the Portuguese forum, I realized we probably do need more options on the German side, but I didn't realize there were so many. And I still would never have known instinctively whether, say, something like LEO felt more like an abstraction, an institution, a communications platform, or a group of people.
However, yes, I do think, or at least hope, it's possible to at least attempt an example or two of each of those. It may not be possible to do more than hint at the range of options, but for people wanting more information, this very thread could be linked if that link function still exists.
Perhaps someone could even give a heads-up to whoever on the LEO team might deal with the same question in the Spanish and Portuguese forums (and anywhere else needed).
In general, I feel like giving the case hardly ever hurts, and often helps. LEO isn't a zero-sum platform -- space isn't an issue.
Especially in this case, though, I don't think it's clear at all to learners which case to guess. When someone comes to a new place, and is standing outside, and someone inside says 'Welcome!' before the person comes in, I think of that as a movement from one place to another (*ahem*, also since we use the preposition 'to'). So actually it wouldn't have surprised me at all if German had conceptualized it as accusative. And in fact, one of the reasons I just looked this up was that I realized I wasn't sure which case it was.
Thinking about it now, maybe it goes back to the underlying grammar or etymology being something more like the Romance languages or the English 'Well come' (cf. 'Well met'), where the past participle assumes that the person is already there, inside. (In German that might be something more like 'Wohl eingekommen'? Or some other prefix to match ...)
But all that is part of why learners have to think about it consciously, or look it up, and not just guess. And all the more so if there are six different prepositions depending on context.
Sorry, I didn't mean to open a can of worms, but I still think we can figure out some way to at least add more than is currently there, namely, nothing. It really is a phrase that a lot of language learners are likely to want to know. (-: