A bigger indirect is usually the right solution, but if you're running out of raw BTUs for space heating that won't work as well if you have multiple wintertime guests.
Some of the Korean condensing stainless tankless units sold under HTP or Noritz nameplates are good values, as are some of the Naviens. But Rinnai has the better rep for reliability and support. A tankless never runs out of hot water, but has a limited gallons per minute rate. In this climate you'll need to max out and go with a 199,000 BTU/hr unit if it has to support more than two simultaneous low-flow showers.
If it's showering performance (and not tub-filling) you're after, a drainwater heat recovery heat exchanger at least 4" x 48" (taller is better) can more than double the capacity of the indirect when there's an 80K boiler backing up the indirect, and doesn't use any fuel.
Shorter versions might do it too, but at 4" x 48" and up you're in the ~50% heat return, cutting the BTUs pulled from the tank during every shower in half. I don't think there's a MassSave rebate for them, but direct from the manufacturer you're looking at about a grand, and it's much simpler to install than a tankless.
They do nothing for tub-fills, since the drain has to be flowing at the same time that the tap or showerhead is running to work.
They also need to be mounted vertically to work, which is sometimes a limitation for first-floor showers over shallow crawlspace foundations (or slab-on grade.) (A 3" diameter PowerPipe can just squeeze into a 2x4 stud bay for second floor showers, if there's no space under the first floor and no fat plumbing chase for a 4 incher.)
It doesn't have to be right next to the water heater either, but if the main drain is far from the water heater it means more plumbing to install it than if they're reasonably close.
Ecoinnovation's Theromdrain and Renewability's Powerpipe drainwater heat exchangers are on the pre-approved MA plumbing list, and would not need to have a variance to install. It's not clear if Ecoinnovation still sells direct (they used to)- it may be distribution only, but Renewability units can still be
purchased onsie-twosie directly , or for a bit more money through the big orange box store.