I'm always a big fan of feeling connected to your previous games.
For example like pokemon bank.
Are there any rumors that we will be able to visit our NL town from our island?
Very, very unlikely. With the graphical change, it'd be even more difficult to pull off and the assets will be different and it's not so simple as just swapping them out.
Probably not that necessary to include. Don't get me wrong, it would be a nice feature, but I would rather have them spend that development time on something else. After all, I can always visit my New Leaf town on my 3DS.
I don't believe the Switch and 3DS can connect directly. The reason I believe this is that in order to transfer a Mii from the 3DS or Wii U to the Switch, the Mii has to be registered to an Amiibo (Not including the Mii connected to your NNID, of course). Also the fact we will still have to use Pokemon Bank.
So what I'm saying is, if they wanted to make New Horizons connect to New Leaf, it would not be simple. I'm not really sure if they'd want to go through the trouble to do it either.