Schemas are an optional part of Solr. Pretty sure that the default schema.xml has an example of a catch-all field definition, if you use that it will automatically deal with any key you want to throw at it.
Of course you need to specify one field type (analysis stack) to apply to all, but I don't know how you expect to avoid that - gonna have to express that metadata somewhere if you need more complex behavior.
Personally I think the _d, _i approach is ok, suffixes aside - complex field analysis options w/o a schema.
The worst part is that I have no idea what benefits schemas are supposed to bring me. The documentation vaguely promises that schemas "can drive more intelligent processing", but I have a feeling I could get that more easily without schemas. It also tells me that "explicit types eliminate the need for guessing of types," but only, apparently, by requiring users to understand and remember them.