Since the job listings mention online service experience I am thinking that they are looking into having some form online/multiplayer features. That said I think they would know very well that forcing players to be online at all times is what killed SimsCity and they had to scrap that feature from TS4 Olympus as a results but I do think they still want to add it in some form. Which is why I think they plan on having TS5 be both solo offline as well as multiplayer online. The offline play would function like it does in all sims game. Multiplayer would be optional and I think the best way to do it would be to make it where players make online groups. Kinda think of it like how in minecraft players can be in worlds that only they and their friends have access too. So I can see the subscription service being for multiplayer. Now whether this online subscription service would be separate like paying for a GTA online service or just be tied into EA Access kinda like how in Animal Crossing if you want to play with other players you have to pay for Nintendo's online service. I think that would probably be best. This way even if you aren't interested in online play but you use EA Access you have the option to look into the multiplayer feature or if you do want to play online you also get access to other games provided in the EA Access.
The other potential reason to have a subscription service would be subscription to have access to all packs without having to pay for each one. The issue with this is what if you cancel your sub do you lose access to all those packs or would there be a reward system in place? Like if I am subbed for 3 months I get the first EP so now it mine and even if I cancel my sub I still own it. This essentially makes it rent to own.