Yeah, maybe it wasn't clear because I talked about the bigger start seats most. But that is why the bigger starting seats was a maybe for me.
I think just making the expansion cheaper is the best and easiest solution. It goes faster as you can build more seats at a time (although we might have to reduce building times a bit as well), while always having an advantage if you started earlier as you would still have to put in all of your money for a while. The advantage is just smaller.
The arena seats starting point is something you could do but I think with this isn"t necessary. It is a maybe and only with a small increase. We could fix what you said by giving players a free upgrade as an investment in your club, like someone already suggested a few comments back, so you can get that money and spend it only to the arena. That way you would also still keep a bit of an advantage.
But just to be clear, I would like the part to just cheapen the expansion cost the most.