Idea for ADDING FIELDS:

"+" icon to the right of headers

Click => adds 1 field with auto name
Right click => menu (add field, add field with name, add multiple fields, add formula field)

ADDING RECORDS:

Show additional blank row at bottom, with "+" in row header:

Click => adds 1 record
Right click => menu (add record, add multiple records)

e.g.:

Field 1 Field 2 Field 3 ... Field 4 [+]
1 data data data ... data
2 data data data ... data
3 data data data ... data
... ... ... ... ... ...
49 data data data ... data
50 data data data ... data
[+]

Then do away with the prompt on "New file" for numbers of records/fields.