Probably one after s29 to make altar permanent. Might need some development. And perhaps to make some things from s29 permanent.
Perhaps some changes to the recycled s30 season theme.
Future patches may have balance changes, updates to their season themes etc.
Nothing big or major is to be expected. Though they may surprise us…
I am assuming beyond s29 we will have even fewer seasons per year and those will probably be recycled from previous seasons.
I don’t think the balancing stops, so even if the sesons recycle there will be new patch notes with fixes and balance changes.
As for Heroes of the storm the news about maintnance mode appear in three days before the final content patch.
Bold assumption, considering it barely happened even with active development.
We had patches without fixes or balance changes? 
I think we might still get balance changes, just no more set reworks so the awful ones will stay that way, and even balance changes few and far between.
I mean even one person with not that much game knowledge can bring in some balance changes based on just adding X% higher multiplier for the desired GR so maybe we see that every now and then?
Or hey maybe they just full on recycle patches and every now and then you get an old insane Inna’s or Firebird’s. Easy way to get people to not feel like the balance is set in stone.
The so-called balance we got always favored one or two builds over all others (e.g. Tal Rasha, Raekor) and when they noticed those builds were too OP they made them noncompetitive (Inna’s, GoD).
So there was never true (numerical) balance between sets of a class or between classes during active development.
Why would it be suddenly different in maintenance mode?
I dunno man. I mean even D2 still have “best” builds. It hasn’t been touched for long time. I don’t think they will “balance” (destroy) builds forever in D3 either.
Ooohh now that you mention it Seeker of the Light still needs a huge buff or rework.
Many sets need number tweaks.
Rage and dmkt delivered the analysis to them on a silver platter and yet nothing was done.
That’s the part that irritates me about this game’s balance. Yes, balancing isn’t always as easy as just “More damage” and it can be very complicated in some games but when you have “Each subsequent level requires fixed percentage extra damage, this build is 5 levels weaker than the top” it is painfully obvious that you can make some quick and easy adjustments. And then so many people literally do the math for them as well.