Now that the recommendations of the report of the privileges committee have been adopted without amendment, Boris Johnson will receive the only sanction left available to the Commons now that he has resigned as an MP; the removal of his entitlement to a former member's pass. Former members may apply for this pass, which allows them limited access to the parliamentary estate without the need to be escorted, and the ability to use certain facilities within.
As Johnson has resigned as an MP, the other sanctions which would have been available to the committee - a time-based suspension from the house, or indeed an outright expulsion - were no longer relevant and did not form part of the report's sanction recommendations. The report puts on the record, though, that had Johnson not resigned, a suspension of 90 days from the House would have been recommended, which would have triggered a recall petition under the Recall of MPs Act 2015. This would have triggered a by-election if signed by more than 10% of the electorate in his constituency. Putting this sanction on the record provides at the very least, some sort of precedent for the seriousness and appropriate punishment for future investigations of this type to consider.
Your question talks about a possible by-election; Johnson's resignation makes this not just a possibility, but a certainty:- the writ of election has already been issued in his former constituency of Uxbridge and South Ruislip, and will take place in late July. There is nothing stopping Johnson from running in this election (although he would probably have to do so as an independent candidate, rather than as a Conservative party candidate).