I wouldn't be surprised if some of the Luton services are also shuffled about. Don't just expect Stansted to be affected. Just look at what launching LGW-ATH resulted in the eventual loss of the LTN-ATH. All I'll say is watch this space.
One thing Stansted has over SEN as a major point (and LTN for that matter) is a decent sized runway that can and does handle large aircraft up to 747-800/A380 to far-flung places. Certainly no chance of EZY transferring any A320's or DLM, BJV, FUE etc...
Both airports it should also be noted have very different catchment areas, tho granted some do overlap.
As per norm the bottom price in the airfare wars will play it's game in 'Jo Po' deciding where they actually fly from.
Good luck to SEN & STN & EZY for the future!