For second person plural informal, I propose "thee-all" (the "ee" can be elided depending on prevailing local custom, yielding "th'all").
Except for Jessica's comment on Quaker usage, most of this thread seems to focus on "standard" English usage of the Elizabethan era, as exemplified by the works of Shakespeare and the King James Bible, but the texts here on the mudcat are in a wide variety of dialectal variants, rarely the London (or New York) proofreader standard. And the fact is that here and there throughout the "English-speaking world" there are or have been dialects that have used most all conceivable configurations of the ø/thou/thee/ye/you/ø pronoun set (plus various additions like y(ou)'all and youse. But as a linguist type I must urge that "nominative" and "accusative" be defined by function, not form. Thus "Thee are way off-base" (vs. "standard" "Thou art...") doesn't involve using "accusative 'thee'" as a subject, but rather using "nominative 'thee'".
Pedant and more