Interesting altcoins that test + refine ideas first proposed for Bitcoin:

Zcash: Zerocash, soft fork proposed for BTC in 2014

Litecoin: MimbleWimble Extension Block, soft fork proposed by Andrew Poelstra for BTC in 2016

Monero: ring signatures, Confidential Transactions, Dandelion++, Bulletproofs, all of which were proposed individually for Bitcoin

Decred: CoinJoin by default, smaller blocks & stable Lightning Network

Bitcoin Cash: dynamic block size, OP_CAT + a bunch of restored OP codes from the Satoshi era (AND, OR, XOR, DIV, MOD, MUL, LSHIFT, RSHIFT)

What else?