I think the OP wanted to mix a seeded wallet with watch only addresses
Exactly. That would be nice feature if it is not possible now. Removing importing of private keys is understandable due to possible losses when recovering from seed (and anyway it should be possible fmpov). In case of imported watch-only addresses you are only could lose those watch-only addresses, no big deal.
Now that Bitcoin supports "pruning" (with a wallet) perhaps the OP should consider using Bitcoin Core instead of Electrum (that should let you create a mixed wallet although you would lose the ability to restore from a seed so you'd have to import all the private keys into a new Bitcoin Core wallet and then be sure to create regular backups of that wallet).
As I remember import of watch-only addresses is in core since some 0.10.x release. My critical requirement now is
my own one private remote full node, I want to stop wasting my space, RAM and CPU on all my laptops and desktops. As I know Electrum is the only solution, if you know alternative please let me know.