By "Bitcoin Core" alternatives I mean full validating clients that follow consensus rules (do not fork the blockchain... because that would be an altcoin).
According to current groupthink, perhaps. After all the years of people claiming this, I'm still not convinced. Surely by definition, it's not an altcoin
until it's on a separate blockchain? If no fork has occurred, it's simply an alternative client which follows the current rules but proposes a potential change to consensus rules
if enough people run the code.
People might not like the proposed changes in the client, but it's still disingenuous to call them altcoins prior to any fork. If or when a minority fork takes place,
then you can call it an altcoin.
If the intention of the new client is to facilitate a consensus breaking fork, then it at least 'intends' to be an altcoin even if is still conforming to the current consensus rules.
Maybe something like proto-altcoin or something might be a more descriptive name. There's probably better names though.