As long as you're winning games, the ROI will be quicker. So a staggered approach between expanding the arena and improving the team seems good.
Speaking of ROI, it's hard to tell about the draft. We don't even know if Utopia will have a National Team, and it'll take forever to get a Utopia-player-only team good enough to where merch bonuses matter. Then again, if we do have a Utopia NT, their will be a very small pool of players to choose from, increasing the likelihood for drafted players to make it.