I agree with a successor to Sega Ages and Sega Mega Drive Classics with the option to purchase additional games but that cross-accounts/account merging is kinda hard for the mobile games without messing anything up.
At least I wish their next mobile games could use fixed SEGA account.
I use facebook account when I first played Sonic Dash and when they kinda forced me to switch to google play account and I did it, I lost all my characters and had to start from zero again.
Well, there are a few Sega games out there with independent accounts. Just if Sega wants to make a big deal with Sega Accounts, it should be a catch-all account for many of their IPs.
I don't have a Nintendo account, but I imagine there are a few fun things to do with it. Such as Mii Studio. Would be nice if Sega Account had fun things to do than just DLCs and deals. I've yet to find incentive to sign up.