Not a suggestion or a request or anything, just something I seen which was pretty cool. We were playing around with Sims 3 modding and it works in a quite funkily way to provide plug-in support to Mac + PC.
Basically they have Mono, the cross platform .NET framework, along with their native engine, to allow for C# DLL plug-ins to be written that work on both platforms. It works really well and it's just occurred to me if, in the distant future, your engine plug-in architecture was mirrored in a Mono interface, it would allow for cross-platform engine plug-ins to be supported in a next generation language, and also in the same language as editor plug-ins would be written in.
Not even sure if this is the way you're going, or whether you're trying to faze out engine plug-ins for more powerful module scripting, which is by nature cross-platform, but just thought I'd mention it. :)
The Mono (or .NET) framework is huge, and so I don't think it's something viable to require or distribute with AGS games. With a game like The Sims where you have to install a CD anyway, it's not an issue -- but I don't think it's something that could really be done for downloadable games like AGS ones.