FFSG
FormulaFolios Smart Growth ETF
stock BATS

If you are sticking with the US market, you really can't get more diverse than just picking ITOT, VTI, or SCHB, unless maybe you add a microcap fund or a different asset class (bitcoin, gold, bonds).
So, you could add an international ETF such as DFIV, FIVA, IVLU, or VYMI. That would certainly bring diversity.
You could also decide there is a certain sector of the market, or a certain class of company, that you want extra emphasis on. This is less diversity, not more, but if are really confident in, say, large US companies in general, you could add a holding like SPYM, SCHX, or VV. Or if you really want extra emphasis on small companies, FESM, FFSG, or SMLF. Perhaps you want to make an extra bet on large companies with strong growth characteristics: look at GARP, VONG, ONEQ, or VOOG. Or you are more attracted to large companies with value characteristics: look at FDRR, EPS, FVAL, ILCV, or VTV. Maybe there is a sector of the economy you think has great long term prospects. If that sector is tech, look at VGT or FSEC. Industrials? FIDU. Utilities? FUTY. You have all sorts of options, just take some time to learn about what they represent and make a thoughtful choice that you have real conviction in, not just going with a bunch of names that randos on social media have suggested :)

If you are sticking with the US market, you really can't get more diverse than just picking ITOT, VTI, or SCHB, unless maybe you add a microcap fund or a different asset class (bitcoin, gold, bonds).
So, you could add an international ETF such as DFIV, FIVA, IVLU, or VYMI. That would certainly bring diversity.
You could also decide there is a certain sector of the market, or a certain class of company, that you want extra emphasis on. This is less diversity, not more, but if are really confident in, say, large US companies in general, you could add a holding like SPYM, SCHX, or VV. Or if you really want extra emphasis on small companies, FESM, FFSG, or SMLF. Perhaps you want to make an extra bet on large companies with strong growth characteristics: look at GARP, VONG, ONEQ, or VOOG. Or you are more attracted to large companies with value characteristics: look at FDRR, EPS, FVAL, ILCV, or VTV. Maybe there is a sector of the economy you think has great long term prospects. If that sector is tech, look at VGT or FSEC. Industrials? FIDU. Utilities? FUTY. You have all sorts of options, just take some time to learn about what they represent and make a thoughtful choice that you have real conviction in, not just going with a bunch of names that randos on social media have suggested :)