Greater Conjured Block would be a great(?) spell that acts like a combination of place and conjure block, in that it makes a conjured "ghost" block that mimics the original in mining speed, blast resistance, etc, but doesn't use up the block, probably doesn't have any functionality if the original is a block entity or whatever, and doesn't drop anything when broken.
I came up with this idea while discussing using conjured blocks as base-building blocks, which would be both laggy and quite fragile.

Consider adding some sanity checks to the values of mining-speed/blast-resistance/etc, so that they're capped at what the best normal vanilla blocks have, or maybe a little higher. That way we don't have a situation where some mod adds The Best Block Ever and it gets used as a bedrock-like greater conjured block.