I would be the client doing oscillation in the pavement, however, I only had that identity for a week.
So I took up something like extensive gardening, scrapbooking, or something obvious to someone like Sherlock, dress differently and live like someone else for a week, someone with boring and irrelevant case to lead him to wrong deduction and dismiss me as just another client. With that, I'd gain access to 221B without suspicion and plant a sophisticated pinhole camera on his Belstaff coat.
While previously I had stolen multiple precious artifacts and placed them in multiple locations, leaving easy enough clues for Scotland Yard to find them, every single one would come with note 'Get Sherlock', and the last artifact contain longest note that all the occupants inside a house would be blown to pieces if he failed to get them on time, the clues had been given.
There would be quite a number of similarity with the stolen artifacts, for example the country of origins, the rare precious gems that appear in each artifacts, or those artifacts once belong to a single collector who led shady live, all to falsely throw Sherlock into multiple frenzies of finding the clue to the house.
All the while the artifact's locations actually form a constellation, and all the coordinates are actually some code when it's translated in language that both Sherlock and Mycroft fluent with. When the code is cracked, it is actually an anagram puzzle that tell alphanumeric with combination of roman numeral of an address. But of course, Sherlock wouldn't have enough time and the last thing he'd hear from me is my voice on the phone telling he is too late and sound of explosion, quite mild explosion because it only target the occupants and not the house, therefore it wouldn't create too much suspicion and get into the news immediately.
Well of course, Lestrade Sherlock John would still try to find the victim even when it's already too late, hoping to catch me and get the remains of the victims. And he would crack the puzzle and find the house with the constellation sign in the front door. They'd enter the house and expect the worst, but it's empty except for a new Belstaff coat with message 'Just want to say Hello, Sherlock. Sorry I ruined your coat' next to the compilation of footage from his camera (yah, there'd be enough time for that
Meanwhile, physically I'd follow John around, who mostly would run around with Sherlock and investigate away. I'd disguise as ASBO officer and keep issuing him ASBO for things that Sherlock would do and thing that I'd plant.
Well, it's probably way above my capability (you think?!!!??
But hey, if it's alternate universe, I want it to be fun and over the top, otherwise I wouldn't want to go.
And this could be fun bonus episode without actual villain, and I'd probably have special place in Sherlock's mind and waste millions of taxpayer's money and authority time.