Here is the best post on IRA maximums. WCI writes for a doc audience but it applies to all.
http://whitecoatinvestor.com/multiple-401k-rules/You can have more than one IRA.
If you own one or more businesses (including are a partner of some form of professional practice) you can contribute up to $53k ($18k plus 18% profits of business is the calculation for limits) per unrelated employer via a SEP-IRA or similarly configured retirement account. Here is just one of many posts on the subject.
http://whitecoatinvestor.com/beating-the-51k-limit-friday-qa-series/Note this is from a few years ago so the limit has increased. If you are over 50 yo you can contribute more!
On top of this you can do $5500 per person (2 if married) to Roth IRA. You could contribute this amount to a traditional IRA as you suggest. However, I believe that you cannot deduct a traditional IRA from your income thus making it useless because you are paying taxes on it anyway, so you might as well get the Roth advantage. Using a Roth is advantageous over putting into a fully taxed brokerage. (check your tax returns to see if you are effectively deducting this amount. If not, it is a waste of time to have it in traditional IRA)
A married person with 2 profitable enough businesses could contribute $106k per year if one person is employed by each business plus $11k contributions to Roths (likely by backdoor). If you could reasonably say that both spouses are employed at both businesses you could do $212K total SEP IRA plus 11K Roth!
You could roll all of your old 401(k)/IRAs into on rollover IRA (not Roth) just for simplicity. You can also roll over to Roth if so desired by paying your current marginal tax rate on the amount converted. I don't know why one would roll into a 401(k) because you generally have much less control of a 401(k) and much higher fees than say a Vanguard or Fidelity rollover IRA account.
You should not roll over to Roth unless you can lower your marginal rate to lower than what it will be in retirement (taking a year off for example). You can also roll over to Roth if you need the money without getting hit by the 10% penalty because the rollover becomes you basis in the Roth account (basis can be withdrawn penalty free - there is a 5 year limit from when you opened the account for withdrawing the basis).
This info is all factual to the best of my knowledge. I hope it helps and correct me if I am wrong.
