most subscribers will get a extra one almost strait up with bronze tier reward, i think as you sub you go up reward tiers, if they have it like that once you've subed youll have 3 then 4 within a month or 2 plus + for every char you get to max level.
have char slots cost a few dollars on store, not very much but a little so people dont just create massive stables to run one mission with all chars every day and also because in a super hero game people will buy a extra char slot or 2 to level chars then sub to get more char slots and finish leveling them that month.
so keep the initial numbers relatively small to start(this also helps a little with controlling bot population if you institute a decent account registration filter to filter out bots spam emails) and youll create a synergistic cycle on cash shop that will tie into people nearing max level in free mode and buying in for sub then more char slots to repeat the initial process to get them into the habit of subbing and buying char slots