Have a real project, that's the best thing they can do.
Look at Roma and Napoli, they've spent sustainably and they've built very good squads even if they've had to sell some of their stars in order to balance the books. It didn't happen overnight, but it can and does work if you put in the necessary time, effort and funds.
Neither Inter nor Milan seem to have any kind of plan on how to make their teams competitive, it's like the money was taken out from beneath their organizations, and they can't seem to function without that same financial capability. There's definitely a vast middle ground between spending boatloads of money on the best players and trying to pick up bargain bin player for cheap or free. You can't run a club of their size on a shoe-string budget, you're going to have to make some expenses but make smart choices for smart money.
What's necessary is just a fundamental restructuring and embracing the new reality, make a clean break with the past and try to be great on different terms.
Start with clearing out the deadwood, which they're both saddled with significantly. Have at least a 5 year plan, prepare your club and your fans to be patient, let the club rebuild slowly, set modest targets and steadily increase them with incremental progress. Have a real budget for transfers, renew the team in a systematic and sensible way, instead of trying to find stop-gap players who from other teams. Yes, sometimes finding those opportunities will help, but you can't build a complete team like that, there has to be a methodical approach. Invest in scouting and youth, even if you can fill a quarter of your squad and even one or two starting spots in your line-up with a few youth players, it's worth making the necessary investment. Figure out how you're going to create more revenue, whether it's through sponsorships, or building and owning their own stadium, and marketing their brand abroad, you have to find a way for the club to succeed as a business. Because if your owner just flat out refuses to pay for it out of pocket, the only way to survive is making sure the club operates feasibly and successful as a separate business entity.
Everything we've seen from them since the financial ramifications of their excessive spending came crashing down has just been haphazard and unorganized.