Hi Matthew, keep an eye on how much weight or leverage you can/should/want to apply to the Heritage's focuser in the shape of an eyepiece.
That aside, I'm not sure replacing Plossls with better Plossls will give you the improvement you are after, so here's another vote for ES. The 8.8mm or 11mm 82° for the (approximate) 2mm exit pupil sweet spot perhaps, where contrast, sharpness, image brightness and power that can be had from a given scope appear best balanced for many, myself included. Power is similar to your existing 10mm, it may not even be that much sharper or contrastier, and eye relief won't be much greater either.
But the view is much wider, and therein lies the improvement. I have both the 13 Nagler and 82°11, and the ES (bought from another IISer for a good price) really does deliver >90% of the Nagler's performance (f/6 dob), and that does take into account the slightly higher power. Edit: the 11mm ES has quite a bit more heft than the 13 Nagler. See above re weight in the focuser, same for this next one:
The ES 68°24 could be worthwhile for the low power end, it will stun under dark skies in particular and is unlikely to fail in too many other scopes. It will be a keeper especially if you want to stay with the 1.25" form factor later. Mine replaced the 24 Panoptic I used to have, and that was one great EP. Either of these 24s will give you the maximum true field of view possible with the Heritage's 1.25" focuser. So would a 32mm or 40mm 1.25" Plossl, but those would give too bright a background at f/5 under your skies to be much fun. The 28mm Edmund RKE is probably a bit too long for the same reasons, otherwise it's an absolute cracker.
High power - a 4mm Plossl sounds torturous but I can relate. If it's of decent quality it probably won't show you much less than significantly more expensive units. Had to look up the Omni -
a 4mm Plossl with an eyecup 
. On paper, this suggests you'd have to be
this guy to fully use its field
Anyway if it works, it works. If not, your most likely improvement will be longer eye relief rather than better image quality, unless you want to spend big money. Perhaps an old Radian? If your PL's image isn't so great the only compromise I can think of would be a mid tier 4mm ortho seeing you are not deterred by short eye relief and/or small useable field.
Edit: Currently still available clones of the TMB Planetary also come to mind, but the quality is all over the place so you may get a lemon.
I'd buy used, right here on IIS whenever possible. Astromart is great too but they do charge a fee for the privilege. For new, I'd have zero apprehensions buying from Agena or OPT, excellent service. Buying local is great but if no Aussie is going to bring them in for you that Aussie might as well be you
