Monero has already made substantial changes to its codebase. Will the RingCT fork make it harder for other CryptoNote coins to merge improvements made to Monero?
Does every major fork make it more difficult for downstream Monero forks (like Aeon) to merge improvements made to Monero (such as LMDB now and soon RingCT)?
Will upstream merges (such as pruning or PoW changes made by Aeon) become more difficult for Monero to implement in the future as its codebase becomes more and more different?
Yes, it will. Every time a substantial change is made in one fork, the other forks will find it more difficult to port anything they might want where it conflicts.
I'd say the main change from the point of view of mergeability between forks was the LMDB change, which rewrote a large part of the blockchain handling.
Cryptonote has also changed a lot since Monero forked, and it would also be difficult for Monero to merge code from newer forks.
I don't think there's been a particular desire from other CN projects to merge code from Monero, however. Some things were merged (payment id related code, some fixes), but I don't think it was that much. Same as Monero did not merge XDN's messaging, for instance.
The work done by Shen is completely opensource: https://github.com/ShenNoether/RingCT
Nothing is barring other projects to fork it and include in their code but I expect AEON will have an easier time since its the "cleanest" fork from Monero, ever since Monero forked from Cryptonote/Bytecoin quite a lot of code change went there.
