You could always make it so that maxing one guild could let one join a
selected set of others. This is a good enough idea I think it was implemented
at one time. Currently the only guild like this is navs. Maybe you could even
code some equivalent five level guild for tanks if you don't want to allow a
huge amount guild versatility. To be honest on HC where you can join all
guilds, many of the combinations weren't very tempting. Many of the guilds
have skills that are at odds with each other, and with the new spell
disruption tune, again problems arise for blaster tanks. Certainly there are
issues, but they aren't quite as huge as some imagine.