A while back I came up with the idea to carve out 4096 code points in plane 14 (Supplementary Special-purpose Plane) for super-surrogates, and use three such surrogates (1 initial, 2 extension) for codepoints beginning from U+110000. If done properly you get unlimited range and self-synchronizing, at the expense of needing 12 bytes minimum per codepoint (more if you want it truly unlimited), but I figured the demand for UTF-16 would be low enough by the time it's needed that it's a workable tradeoff.