I dont think that the arena should be changed a lot and I dont like the idea of purchasing some temporary attractions to boost your short term income.
However I tthink there should be the possibility to buy or build some extras that cost a lot but will give a nice return in the long run. A possible expansion would allow you to rent your arena x times a season for a congress, concert or some other event. This would require a stadium with 10k+ seats for example.
Training facilities would be nice too, a possible effect of those could be that the cap of a player would raise one level so you could train them further and better. this would benefit smaller countries that dont draft hall of famers all the time and have to rely on a much smaller player base.