I dont see how you would be able to give them your own flight plans. Computed flight plans need to have a navigation database, imput of winds aloft and the aircraft data from the manufacturer. It then needs to be able to optimise on a selection of routes. You would not be able to do this by yourself. (It gets more complex with crossing oceans so including tracks, and Europe needs to be RAD and CDR compliancy). There are many flight planning providers already on the market for all sizes of operation, so cannot see that you would be able to offer anything comparable or at all to be honest. You may be able to set up to provide them with weather charts or METARs and TAFs but again most operators already have their own services set up and with the abundance of information on the internet you would be unlikely to be able to be competitive. (IMHO)
If you only want to offer a third party's products then you need the permission of that company and not EASA.