there are so many ideas depends how can you get traffic for you apps.
for example
i recently made a app android right now EMoji type app I hope you Know Emoji icons for example this kinda app (https://play.google.com/store/apps/details?id=com.pengpeng.coolsymbols&hl=en) .. so from my app you can send share the icons on whats up, email, Facebook , Twitter, instagram , flicker, the icons are v.cool my designer took 6 month to develop those icons or images like emoji (funky icons, love icons, hate icons, mody icons)
for adverting I pay 6000 per month to facebook i am member of multimedia community page if any page whos users are in millions facebook charge for that page like you can advertise whatever you want
from that page i Generate traffice for my apps other different techniques also including SEO and keyword search on google store.
soon gonna upload on Apple store next version my future plan are Emoji+Customize icons app.
i shared idea how do i get traffic hope it helps.
Two major things are important your idea should be good and you must know how to generate traffic for your app then later you can put Google add ., banner , Facebook page , and once your one app successful then you can advertise your next app from that app.
hope you understand what i am trying to explain.
Thanks,