There are many ways to continue your search. Have you ordered and looked at LDS microfilms? Order the microfilms of Kollerschlag. The next targets for you should be the birth record of Anna, the death record of Rudolf, and the birth record of Rudolf. The birth record of Anna (your grandmother) will show, amongst others, the maiden name of her mother. The death record of Rudolf will show whether he left a widow and possibly her name. You could also look through the marriage records of 1880 till 1900. However if Rudolf and Franziska married somewhere else (not Kollerschlag), you would need for clues.
As an alternative to ordering the microfilms, you could wait until the duplicates are digitized and added to the Matricula records of Kollerschlag.