Barcelona have huge financial problems (if I'm not mistaken they still have a debt to Messi even now) so there is no chance that Messi will return to Barcelona. In my opinion, Messi can go to MLS and earn a lot of money there in a season or two, and in his position it will be the best solution, as it was already written above, age makes itself felt and it will soon become more and more obvious that he does not correspond to the level of a top club.
They are not "that" much in financial trouble anymore, plus what people are talking about is Messi getting a huge pay cut just to retire there, all of which could result with something much bigger. I am not saying that will happen, but if he comes in next year taking like 10-20 million euros instead of what he usually does, then I am sure that they could find that money somewhere.
Look at what they have done this year, they have spend a ton of money, more than almost any team in the world, and they could just replicate a small version of that to get Messi, which would make sense since they would financially recoup that back with just jersey sales alone.
How do you determine that they are "not so much" in financial trouble? If Barcelona don't have big financial problems then why do they owe just three players (Messi, Piqué and Busquets) more than 100 million? And these are only the largest debts to the players; there are even smaller ones.
And as you rightly say, they have spent a ton of money this season, but at the same time they risk leaving the Champions League after the group stage. If this happens, their financial situation will be greatly aggravated. In this position, signing Messi would be pure madness.