Sword and Shield will be getting bi-monthly patches starting in December adding a new Legendary Pokemon to be sent via Mystery Gift with each patch, to keep dataminers in the dark each patch will be adding a new Legendary as well as bug fixes.
December = Mewtwo
February = Ho-Oh
April = Mew
June = Celebi
That is all that is planned for now, but Sinnoh remakes are in the works and Shaymin will be added with their announcement.
I know Masuda has said there will be no patches, however the plan has changed since then, do not expect any Pokemon to be added besides Legendaries.

In a Pokemon direct right before E3 2020 will be an announcement for Diamond and Pearl remakes featuring Dialga and Palkia fused with Giratina as their mascots. Time-Warped Giratina, and Space-Torn Giratina. Release is on a soft date of November 13th 2020.

Finally there is early plans of a sequel to Sword and Shield akin to Black and White 2. This could change over time but the current plan is to add in 200 more past generation Pokemon and 10 brand new Galar Pokemon. Plot will be a direct sequel. There is some talk of enhancing the graphics but it probably won't be possible in time.

I have no proof I will give but you will see as time passes that I am right.