Barring some rework of the gym mechanics, this current gym system is here to stay, and it is rather clear in the patterns it brings. You have bulky normal types like Blissey, Chansey, and Snorlax defending, and the likes of Dragonite, Raikou, Rayquaza, and the king himself Machamp going to sweep.

The only prolonged mechanic is if the gym happens to be fed heavily by the defending team, but even then coordination can halt such endeavors. So unless Pokémon Go gets the much needed combat change it deserves, along with maybe some PVP, what will future Pokemon bring to the gym system.

Future Gym Defenders

There have been recent additions to the gym defense meta with Generation 3, namely the likes of Slaking, Metagross, and Gardevoir as potential defenders, with all of them being decently viable to stall a Machamp in its tracks, having access to psychic or fairy type moves, as well as the latter two resisting his attacks. However, for the future generations the pickings get rather slim for defenders, frankly we have the best defenders in the game with the likes of Blissey, Chansey, Snorlax, and a few others.

Not all hope is lost though, with the right moves to complement given stats, as we understand the stats formula now, a few might shine in the defense aspect. This is not a definitive list by any means, as it depends on several factors, as well as excluding legendaries as they have no ability to defend, but judging by the relative stamina and defense stats, as well as relevant resistances to common fighting, dragon, and electric threats, these seem to stand out:

Potential Defenders
Pokemon Attack Defense Stamina Generation
Pokemon GO AlomomolaAlomomola 138 148 330 5
While obviously lacking in the attack department, it has the highest stamina stat of the future generations, as well as a defense stat to rival Chansey, as a 28 point difference. Plus, Machamp only does neutral damage, although this does mean a switch would occur to another great gym generalist, Raikou.
Pokemon GO DrifblimDrifblim 180 107 300 4
While the defense stat leaves much to be desired, the typing, ghost and flying, leaves 6 of resistances, including common attackers of the fighting and ground variety, as well as boasting the second highest stamina stat in future generations, tied with Giratina.
Pokemon GO ThrohThroh 172 160 240 5
It might not be Hariyama, but it can be a good substitute, being only 40 less in stamina, but with 46 more in defense. As with either of these, it will be a type to resist Machamp.
Pokemon GO AegislashAegislash 97 291 120 6
While being more than Mediocre in attack and stamina, the defense isn’t anything to scoff at, as well as it boasting 12 resistances, and weaknesses to only ground, ghost, fire, and dark.

Future Gym Attackers

While the potential defenders might be lackluster, the list of attackers is sure to grow, with some contenders even being able to knock Machamp from his post. There are too many potential attackers to count, not to mention their viability, like defenders, is primarily tied to counting on old moves currently in the game and their access to those moves in the main series. If anything, this is our best guess at the future of attackers. All stats are to be considered, but the most consideration goes into attack, and seeing as how legendaries can attack and participate in raids, they will be included here:

Potential Attackers
Pokemon Attack Defense Stamina Generation
Pokemon GO ConkeldurrConkeldurr 243 172 210 5
Fighting This is a direct, non-legendary upgrade to Machamp, boasting all around increases in stats over Machamp, as well as it should have access to all the moves that Machamp has.
Pokemon GO MamoswineMamoswine 247 157 220 4
Ground Ice One of the many Gen 4 Pokémon to evolve from a previous Generation, Mamoswine vastly improves over Piloswine, being the highest attack Ice type of all generations save the Legendary Kyurem.
Pokemon GO ReshiramReshiram
Pokemon GO ZekromZekrom
Pokemon GO DialgaDialga
302 200 242 4/5
All of these dragons fit the bill, rocking identical stats, filling their respective niches, and having some of the highest offensive stats in the game. Given the right moves, they can kick out known generalists, and even outclass current Pokémon in their niche based on their secondary typing.
Pokemon GO RampardosRampardos
Pokemon GO RhyperiorRhyperior
295
241
114
224
194
230
4
Both are included for their relative bulk, in comparison to their counterparts on the higher rock attacker end, as well as for the monstrous attack stats that can make them formidable even without type advantage. The hope here would be the addition of a new rock fast move beyond rock throw, which neither can use.
Pokemon GO YveltalYveltal  275 204  252 6
Quite possibly the one and only successor to Tyranitar, being able to have more bulk and more fighting power than Ttar as well as possibly having access to Foul Play, a more powerful move than Crunch.

To sum it all up, there will not be too many exciting defenders in the future, however there are going to be a bevy of attackers that can hit a gym, outright replacing current favorites, and allowing for more coverage where there currently might be little. This is of course not a complete list, however these seem to be the current popular stars that might shake the gym meta, and allow for a greater variety of attackers.