What do I get from playing a Seasonal character without RoS?

I have recently started playing Diablo 3 again, and have discovered a "Seasonal" Character choice. Upon investigation, the main focus is to reward the player after achieving certain things within the game. However, 2 of these goals require the Expansion ("Adventure Mode"); which I don't have.

Without the Expansion, what do I get from playing a Seasonal Character specifically (that is not included in the base Expansion like extra classes etc)?

There are only a few things that requires you to play Seasons to obtain:

1. New portraits
2. 1 Additional stash tab
3. New seasonal pet

Since these are all tied to the season journey, which is primarily adventure mode focused, there is no benefit of you playing seasons when you don't own the expansion pack (as you have noted).

Previously, there were season only legendaries but that has been removed since Patch 2.4:

As you may have heard, we’re removing Season-exclusive Legendaries from the game.