I am uncertain of the best solution but we seem to have at least 4 tags that at least touch on the areas you mention above. I don't like monero-level-2 because I think it will lack a clear meaning for many users of this stack.

tx-extra

http://monero.stackexchange.com/questions/tagged/tx-extra

scaling

http://monero.stackexchange.com/questions/tagged/scaling

side-chain

http://monero.stackexchange.com/questions/tagged/side-chain

lightning-network

http://monero.stackexchange.com/questions/tagged/lightning-network

I am uncertain of the best solution but we seem to have at least 4 tags that at least touch on the areas you mention above. I don't like monero-level-2 because I think it will lack a clear meaning for many users of this stack.

tx-extra

https://monero.stackexchange.com/questions/tagged/tx-extra

scaling

https://monero.stackexchange.com/questions/tagged/scaling

side-chain

https://monero.stackexchange.com/questions/tagged/side-chain

lightning-network

https://monero.stackexchange.com/questions/tagged/lightning-network