Optimal grouping requirements are pretty much a player enforced thing.-Develop exp methods that don't use the standard grouping model. Zonewide forced hardraids with afk timeouts, more allowance or ways to mitigate for "suboptimal" combinations.
An example: not super long ago I joined a pets only group. 2 mages, 2 necros, and a beastlord. Yeah we had an empty spot but that happens. We killed monsters, got xp, and some mediocre platinum dollars. Also done a caster-only group in everchill. That was fun too. Was it fast paced and the best xp I've ever seen? No, but it was a less than optimal group, so less than optimal xp is kind of expected.